Refer to these abstracts as:
Author(s) (2004), Title, Eos Trans. AGU, 85(47), Fall Meet.
Suppl., Abstract ###-#.
Melting
Behaviors of Recycled Subducted Crust in the Mantle:
Constraints from Melting Phase Relations of Pyroxenites
Kogiso, T. & Hirschmann, M. M.
Abstract Poster |
The
Role of Garnet Pyroxenite in High-Fe Mantle Melt Generation:
High Pressure Melting Experiments
Tuff, J., Takahashi, E., Takahashi, E.
& Gibson, S.
Abstract Poster
|
A
Global Dataset of Noble gas Concentrations and Their
Isotopic Ratios in Volcanic Areas
Abedini, A. A. & Hurwitz, S.
Abstract Poster
|
The
Chemical Surface Signature Created by Upwelling Plumes
Schmalzl, J. & Hansen, U.
Abstract Poster |
Limitations
on the Estimation of Parental Magma Temperature Using
Olivine-melt Equilibria: Hotspots Not So Hot
Natland, J. H.
Abstract Poster
|
Two-Stage
Melting Of Mantle Plumes And The Origin Of Rejuvenescent
Volcanism On Oceanic Islands
White, W. M. & Morgan, J. P.
Abstract Poster |
How
Many Plumes In Africa ? The Geochemical Point of View
Pik, R., Marty, B., Marty, B. & Hilton,
D.
Abstract Poster |
Recycling
and Mantle Stirring Determined by $^{142}$Nd/144Nd Isotopic
Ratios
Jacobsen, S. B. & Ranen, M. C.
Abstract Poster |
The
Importance of Being Plumes: Entrainment, Isotopes, Melting
and the Vertical Structure of the Earth's Mantle
DePaolo, D. J. & Weaver, K. L.
Abstract Poster |
MiFil:
a method to characterize seafloor swells with application
to the South Central Pacific
Bonneville, A., Adam, C. & Vidal, V.
Abstract Poster |
On
the Zoology of Mantle Upwellings : the case of the Pacific
Davaille, A., Bonneville, A. & Stutzmann, E.
Abstract Poster |
Large
Igneous Provinces, Mantle Plumes and Uplift: A Sedimentological
perspective
Mazumder, R. & Foulger, G. R.
Abstract Poster
|
Heat
Flow on Hotspot Swells Reflect Fluid Flow Masking Potential
Variations in Mantle Heat Flux
Harris, R. N. & McNutt, M. K.
Abstract Poster
|
Cratonal
lithosphere a potential recorder of ancient mantle plumes
Sleep, N. H.
Abstract Poster
|
Diagnostic
Features of Mantle Plume Penetration Into the Lithosphere.
Jurine, D., Jaupart, C. & Brandeis, G.
Abstract Poster |
Revised
Models of Global Plate Motions and Mantle Flow Successfully
Predict the Emperor-Hawaii and Other Hotspot-related
Seamount Chains
Sutherland, R., Steinberger, B. & O'Connell,
R. J.
Abstract Poster
|
Intermittent
Volcanism in the S Pacific: Tracking Persistent Geochemical
Sources
Konter, J. G., Koppers, A. A., Staudigel, H., Hanan,
B. B. & Blichert-Toft, J.
Abstract Poster |
Improved
Absolute Plate Motion Modeling in the Pacific
Wessel, P., Harada, Y. & Kroenke, L. W.
Abstract Poster
|
Paleomagnetic
Tests of Global Plate Reconstructions with Fixed and
Moving Hotspots
Andrews, D. L., Gordon, R. G. & Horner-Johnson,
B. C.
Abstract Poster |
Latitudinal
Shift of the Hawaiian Hotspot: Motion Relative to Other
Hotspots or True Polar Wander?
Gordon, R. G. & Horner-Johnson, B. C.
Abstract Poster |
Fifty
Million Years of Fixed Hotspots: A New Self-Consistent
Global Set of Plate Rotations
Kumar, R. R., Andrews, D. L. & Gordon, R. G.
Abstract Poster |
Is
the Hawaiian-Emperor Bend Coeval for all Pacific Seamount
Trails?
Koppers, A. A. & Staudigel, H.
Abstract Poster
|
Critical
Evaluation of Radiometric Ages Used for Tracking Hotspots
in the Pacific Ocean
Baksi, A. K.
Abstract Poster
See also discussion
by A. Baksi on Pacific seamount ages |
40Ar/39Ar
Geochronology of the Sylhet Traps, Eastern India, and
their relationship to the Kerguelen Plume related magmatism
Ray, J. S. & Pande, K.
Abstract Poster
|
High
precision Pb, Sr, and Nd isotope geochemistry of alkalic
early Kilauea magmas from the submarine Hilina bench
region, and the nature of the Hilina/Kea mantle component
Kimura, J., Sisson, T. W., Nakano, N., Coombs, M.
L. & Lipman, P. W.
Abstract Poster
|
Submarine
Alkalic Lavas Around the Hawaiian Hotspot; Plume and
Non-Plume Signatures Determined by Noble Gases
Hanyu, T., Clague, D. A., Kaneoka, I., Dunai, T.
J. & Davies, G. R.
Abstract Poster
|
A
Combined He and Os Isotopic Study of the HSDP-2 Core
from Mauna Kea, Hawaii
Ireland, T. J., Walker, R. J., DePaolo, D. J. &
Kurz, M. D.
Abstract Poster |
The
Chemical Structure of the Hawaiian Mantle Plume
Ren, Z., Hirano, N., Hirata, T., Takahashi, E. &
Ingle, S.
Abstract Poster |
Extreme
Hf-Os Isotope Compositions in Hawaiian Peridotite Xenoliths:
Evidence for an Ancient Recycled Lithosphere
Bizimis, M., Lassiter, J. C., Salters, V. J., Sen,
G. & Griselin, M.
Abstract Poster
|
He,
Sr, Nd, and U Isotopic Variations in Post-Shield Lavas
From the Big Island of Hawaii - Insight Into Magma Transport
and the Chemical Structure of the Hawaiian Plume.
Aciego, S. M., DePaolo, D. J., Kennedy, B. M. &
Christensen, J. N.
Abstract Poster |
Generation
of Primary Kilauea Magmas: Constraints on Pressure,
Temperature and Composition of Melts
Gudfinnsson, G. H. & Presnall, D. C.
Abstract Poster
|
Geochemical
Constraints on the Enriched End-member of the Hawaiian
Plume: Temporal Geochemical Variation Within the Koolau
Shield
Huang, S. & Frey, F. A.
Abstract Poster
Preprint
|
New
Seismic Constraints for the Yellowstone Hotspot
Dueker, K. G., Schutt, D. L., Yuan, H. & Fee,
D.
Abstract Poster |
Yellowstone
Hotspot Melting And Its Relation To Pre-Existing Crustal
Structures And Great Basin Extension
Glen, J. M., Ponce, D. A. & Sepulveda, E.
Abstract Poster |
Tomographic
Images of the Yellowstone Hotspot Structure
Jordan, M., Smith, R. B. & Waite, G. P.
Abstract Poster |
Reconciling
Observations of the Yellowstone Hotspot with the Standard
Plume Model
Ihinger, P. D., Watkins, J. M. & Johnson, B.
R.
Abstract Poster |
New
Constraints on the Evolution of the Deccan Volcanic
Province, India
Mohan, G. & Ravi Kumar, M.
Abstract Poster |
Hafnium-Osmium
Systematics of Cretaceous Group II Kimberlites from
India
Kent, R. W., Ingle, S., Mattielli, N., Kempton,
P. D., Saunders, A. D. & Suzuki, K.
Abstract Poster
Supplement |
K-T
magmatism of western Rajasthan, India: Manifestation
of Reunion plume activity or extensional lithospheric
tectonics?
Sharma, K.
Abstract Poster |
Implications
for the Emplacement of the Deccan Traps (India) From
Isotopic and Elemental Signatures of Dikes
Vanderkluysen, L., Mahoney, J. J. & Hooper,
P. R.
Abstract Poster |
New
Age and Geochemical Data From Seamounts in the Canary
and Madeira Volcanic Provinces: A Contribution to the
"Great Plume Debate"
Geldmacher, J., Hoernle, K., Duggen, S. & Werner,
R.
Abstract Poster |
Ascension
Island, South Atlantic: Deep Plume or Shallow Melting
Anomaly?
Weaver, B.
Abstract Poster |
Tristan-Gough
Plume: Negative Ce-Anomalies as Evidence of a Recycled
Sediment Component in the Deep Mantle
Class, C. & le Roex, A.
Abstract Poster |
The
Origin of EM1 Signatures in Basalts From Tristan da
Cunha and Gough
Stracke, A., Willbold, M. & Hemond, C.
Abstract Poster |
Isotope
and Trace Element Characteristics of Walvis Ridge Basalts
Argue Against Pelagic Sediment Involvement
Salters, V. J. & Li, X.
Abstract Poster |
Contrasting
Styles Between the Structure and the Magmatism of the
West and South Hatton/Rockall Margins (North Atlantic
Igneous Province)
Gernigon, L., Ravaut, C., Shannon, P. M., Chabert,
A., O'Reilly, B. M. & Readman, P. W.
Abstract Poster |
Plumes
are not what they seem: Physics of big, fat, firm plumes
Korenaga, J.
Abstract Poster |
The
mantle potential temperature anomaly beneath Iceland
is insufficient for a thermal plume.
Foulger, G. R., Vinnik, L. P. & Du, Z.
Abstract Poster
|
Widespread
Synchronous Volcanism Reveals a Broad Galapagos Hotspot
Melting Anomaly
O'Connor, J. M., Stoffers, P., Wijbrans, J. R. &
Worthington, T. J.
Abstract Poster |
Upper
Mantle Structure Beneath the Gal\'{a}pagos Hotspot from
Surface Wave Tomography
Villagomez, D. R., Toomey, D. R., Hooft, E. E. &
Solomon, S. C.
Abstract Poster |
Young
lava fields on the Cretaceous Pacific Plate in the Japan
Trench: Non-hotspot volcanism?
Hirano, N., Haraguchi, S., Yamamoto, J., Takahashi,
E., Hirata, T., Takahashi, A. & Ogawa, Y.
Abstract Poster |
New
insights on the Marquesas volcanic chain emplacement
Adam, C. & Bonneville, A.
Abstract Poster |
Geochemical
Evolution of the Hikurangi Oceanic Plateau, New Zealand
Hoernle, K., Hauff, F., Werner, R., Mortimer, N.,
van den Bogaard, P., Geldmacher, J. & Garbe-Schoenberg,
D.
Abstract Poster |
A
South West Pacific Example of Volcanic Rift Margin Facies
in a Backarc Basin Setting From Norfolk Basin Seismic
Reflection Profiles
Taylor, L. & Muller, D.
Abstract Poster |
Radial
Volcanic Migrations Above Continental Hotspots: Examples
from Arabia and the Pacific Northwest
Camp, V. E., Orihashi, Y. & Ross, M. E.
Abstract Poster |
Upper
Mantle Origin of the Newberry Hotspot Track: Evidence
From Shear-Wave Splitting
Xue, M. & Allen, R. M.
Abstract Poster |
Mantle
wedge perturbation induced by slab detachment and the
Mio-Pliocene bimodal volcanism in the Trans-Mexican
Volcanic Belt
Ferrari, L., Orozco, M. & Petrone, C. M.
Abstract Poster
|
Why
are Low-Ti Basalts of the Siberian Traps Large Igneous
Province Similar to Island Arc Basalts?
Ivanov, A. V., Rasskazov, S. V., Demonterova, E.
I., Yasnygina, T. A., Maslovskay, M. N. & Feoktistov,
G. D.
Abstract Poster
|
Silicate
Veining Above an Ascending Mantle Plume - Evidence from
New Ethiopian Xenolith Localities
Rooney, T. O., Furman, T., Ayalew, D. & Yirgu,
G.
Abstract Poster
|
Antipodal
Hotspots and Bipolar Catastrophes: Were Oceanic Large-Body
Impacts to Blame?
Hagstrum, J. T.
Abstract Poster
|
Mantle
Plume Magmatism on Present-day Mars
Kiefer, W. S.
Abstract Poster
|
V51B-0522
Melting
Behaviors of Recycled Subducted Crust in the Mantle: Constraints
from Melting Phase Relations of Pyroxenites
* Kogiso, T (kogisot@jamstec.go.jp)
, JAMSTEC, 2-15 Natsushima, Yokosuka, 237-0061 Japan
Hirschmann, M M (Marc.M.Hirschmann-1@umn.edu) , Univ. Minnesota,
310 Pillsbury Dr SE, Minneapolis, MN 55455 United States
Ocean island basalts
(OIB) have isotopic and trace element signatures that are
considered to originate in subducted mafic oceanic crust.
Recent experimental studies on mafic lithologies (pyroxenites)
revealed that diversity in major element composition of OIB
can also be explained by partial melting of a variety of pyroxenites
derived from subducted mafic crust. Most part of subducted
oceanic crust has MORB-like (silica-excess) composition, but
the uppermost part of the subducted oceanic crust suffers
extraction of small degree partial melts or siliceous hydrous
fluids during subduction, resulting in formation of bimineralic
pyroxenite which consists solely of garnet and clinopyroxene.
Partial melts from MORB-like pyroxenite at high pressures
have silica-saturated compositions, which have similarities
to silica-rich tholeiitic OIB [1], whereas partial melts from
bimineralic pyroxenite have silica-undersaturated compositions
similar to alkalic OIB lavas [2]. On the other hand, subducted
oceanic crust is hybridized with surrounding mantle peridotite
through mechanical and diffusive interactions with peridotite,
yielding olivine-bearing pyroxenite. To investigate melting
behaviors of such olivine-bearing pyroxenite, we have conducted
partial melting experiments on a mixture of bimineralic eclogite
(B-ECL1) with 5 % Fo60 olivine (bulk Mg\# = 61) at 5 GPa.
The experimental results show that addition of olivine to
B-ECL1 expands the stability of garnet in the residue, resulting
in producing liquids richer in SiO$_{2}$ and alkalis and poorer
in Al$_{2}$O$_{3}$ than those of B-ECL1. At $1600\deg$C, the
degree of partial melting of the B-ECL1 + olivine mixture
is higher than that of B-ECL1, suggesting that the solidus
temperature is lowered by addition of olivine. However, experiments
on a Mg-rich pyroxenite (MIX1G: bulk Mg\# = 79), which has
more olivine component, have shown that the solidus of MIX1G
is higher than $1600\deg$C [3]. These results suggest that
bimineralic pyroxenite can produce larger amounts of partial
melts when it is mixed with smaller amounts of olivine. This
further indicates that upwelling mantle containing recycled
crust with bimineralic composition would begin to melt at
the boundary between pyroxenite and peridotite, producing
strongly alkalic liquids first. [1] Pertermann and Hirschmann
2003, J.Petrol. 44, 2173. [2] Kogiso and Hirschmann 2002,
EOS 83, V71C-12. [3] Kogiso et al. 2003, EPSL 216, 603. Back
V51B-0523
The Role of
Garnet Pyroxenite in High-Fe Mantle Melt Generation: High
Pressure Melting Experiments
* Tuff, J (jtuf02@esc.cam.ac.uk)
, Department of Earth Sciences, University of Cambridge, Downing
Street, Cambridge, CB2 3EQ United Kingdom
Takahashi, E (etakahas@geo.titech.ac.jp) , Department of Earth
and Planetary Sciences, Tokyo Institute of Technology, Meguro,
Tokyo, 152-8551 Japan
Takahashi, E (etakahas@geo.titech.ac.jp) , Institute for Research
on Earth Evolution, Jamstec, 2-15 Natsushima-Cho, Yokosuka,
237-0061 Japan
Gibson, S (sally@esc.cam.ac.uk) , Department of Earth Sciences,
University of Cambridge, Downing Street, Cambridge, CB2 3EQ
United Kingdom
Evidence for the existence
of heterogeneous or 'marble cake' convecting mantle$^{1}$
is provided recently by rare, high MgO ($\sim$ 15 wt.%) primitive
magmas with anomalously high abundances of FeO* ($\sim$ 13.5
to 16 wt. %$^{2,3}$; where FeO* = total Fe as FeO). These
high-Fe mantle melts show a limited occurrence in the initial
stage of magmatism in large igneous provinces (e.g. Deccan,
Ethiopia and Paran\'{a}-Etendeka) and some have incompatible
trace-element and radiogenic-isotopic ratios (Sr, Nd and Pb)
that resemble those of ocean-island basalts. This suggests
that they are predominantly derived from the convecting mantle$^{2}$.
The ferropicrites are mildly- to sub-alkaline and have low
contents of Al$_{2}$O$_{3}$ ($<$ 10 wt.%) and heavy rare-earth
elements (e.g. Lu $<$ 0.18ppm) that are consistent with
the increased stability of garnet, due to the high FeO* content
in the ferropicrite mantle source. It has been proposed that
the source of the high FeO* may be garnet-pyroxenite streaks
derived from subducted mafic oceanic crust$^{2}$. We have
undertaken melting experiments between 1 atmosphere and 7
GPa in order to determine the anhydrous phase relations of
an uncontaminated ferropicrite lava from the base of the Early-Cretaceous
Paran\'{a}-Etendeka continental flood-basalt province. The
sample has high contents of MgO ($\sim$ 14.9 wt.%), FeO* (14.9
wt.%) and NiO (0.07 wt.%). Olivine phenocrysts have maximum
Fo contents of 85 and are in equilibrium with the host rock,
assuming a K$_{d}$ of 0.32 and we believe that the sample
is representative of a primary Fe-rich mantle plume derived
melt. In total, 75 experimental runs were carried out. Melting
phase relations as well as compositions and modal proportions
of all coexisting phases were successfully determined in 60
run products. Phase relations indicate that the ferropicrite
melt was generated either at $\sim$ 2.2 GPa from an olivine-pyroxene
residue or $\sim$ 5 GPa from a garnet-pyroxene residue. A
low bulk-rock Al$_{2}$O$_{3}$ content (9 wt.%) and high [Gd/Yb]$_{n}$
ratio (3.1) are consistent with residual garnet in the ferropicrite
melt source and favour high-pressure melting of garnet-pyroxenite.
The garnet pyroxenite may represent subducted oceanic lithosphere
entrained by the upwelling Tristan mantle plume starting-head.
During adiabatic decompression, intersection of the garnet
pyroxenite solidus at $\sim$ 5 GPa would occur at mantle potential
temperatures of $\sim$ 1550$\deg$C. Subsequent melting of
peridotite at $\sim$ 4.5 GPa may be restricted by the thick
overlying sub-continental lithosphere such that dilution of
the garnet-pyroxenite component would be significantly less
than in intra-plate oceanic settings. This model accounts
for the limited occurrence of ferropicrite magma in the initial
stage of continental large igneous provinces and its absence
in ocean-island basalt successions. $^{1}$ All\`{e}gre {\it
et al}., Philosophical Transactions of the Royal Society of
London A297, 447-477 (1980). $^{2}$ Gibson {\it et al}., Earth
and Planetary Science Letters 174, 355-374 (2000). $^{3}$
Gibson, Earth and Planetary Science Letters 195, 59-74 (2002).
Back
V51B-0524
A Global Dataset
of Noble gas Concentrations and Their Isotopic Ratios in Volcanic
Areas
* Abedini, A A
(aabedini@usgs.gov) , U.S. Geological Survey, 345 Middlefield
Rd., Menlo Park, CA 94025 United States
Hurwitz, S (shaulh@usgs.gov) , U.S. Geological Survey, 345
Middlefield Rd., Menlo Park, CA 94025 United States
The extent to which
ocean islands are derived from the deep mantle (mantle plumes)
or from chemical heterogeneities embedded within the mantle
convective flow has long been debated. Noble gases have unique
properties that provide significant information regarding
this debate and make them important geodynamic tracers. The
study of noble gas isotopic compositions in active tectonic
and volcanic areas is central to understanding the origins
of major volcanic anomalies. For example, helium isotope composition
is considered to be the most unambiguous geochemical indicator
of a lower mantle plume component in surface rocks, and its
variability is often taken as the strongest evidence for a
layered mantle. We have compiled all the published global
noble gas data from MOR, ocean islands, seamounts, and volcanic
arcs, extending existing datasets which are limited to samples
from oceanic rocks (Farley and Neroda, 1998; Graham, 2002).
Our data set contains information on helium, neon, argon,
and xenon concentrations, as well as their isotopic ratios.
Where available we also included the isotopic ratios of lead,
strontium, neodymium, and carbon. Overall, there are more
than 5,000 entries in the database, which is sub-divided both
by material sampled (e.g., volcanic glass, different minerals,
fumarole, spring) and by tectonic setting (MOR, ocean islands,
volcanic arcs). Our extended dataset is consistent with earlier
studies and shows that helium isotope ratios in MORB glass
in the Pacific, Indian and Southern Atlantic Oceans have a
sharp peak between 7 and 9 RA. The large peak in MORB samples
correlates with the maxima in samples from volcanic arcs,
probably implying that the upper mantle has a uniform helium
isotope ratio. In the North Atlantic there is a broader distribution
with a maximum at 11 Ra. In contrast, helium isotope ratios
in ocean-island basalts (OIB) are highly variable. As noted
by previous workers, the helium isotope ratios in OIB have
a large peak at 8 RA and a second peak at 13 RA, separated
by a pronounced minimum at about 10 RA. In most cases, higher-than-MORB
He-isotope ratios coincide with deep mantle plumes revealed
by seismic tomography (Montelli et al., 2004). The database
will be available through the World Wide Web and will allow
examination of some unresolved scientific problems. Farley
K.A., Neroda E., Ann. Rev. Earth Planet. Sci., 1998. Graham,
D. W., Rev. Mineral. Geochem., 2002. Montelli, R. et al.,
Science. 2004. Back
V51B-0525
THE CHEMICAL
SURFACE SIGNATURE CREATED BY UPWELLING PLUMES
* Schmalzl, J
(joergs@uni-muenster.de) , University Muenster Institute of
Geophysics, Corrensstr. 24, Muenster, NRW 48149 Germany
Hansen, U (hansen@earth.uni-muenster.de) , University Muenster
Institute of Geophysics, Corrensstr. 24, Muenster, NRW 48149
Germany
Convective flows govern
much of the dynamics of the Earth. Examples of such flows
are convection in the Earth's mantle, convection in magma
chambers and much of the dynamics of the world oceans. Nowadays
these time-dependent flows are often studied by means of three
dimensional (3D) numerical models which solve the equations
for the transport of heat and momentum alternatingly. These
flows are often driven by a temperature difference. But for
many flows there is also an active or passive chemical component
that has to be considered such as entrainment of material
from an underlying layer. One characteristics of these flows
is that the chemical diffusivity is very small. Implementing
such a chemical field with a very low diffusivity into a numerical
model using a field approach is difficult due to numerical
diffusion introduced by the Eulerian schemes. We have implemented
a tracer-mesh method which tracks only the position of the
interface between the two different components. While the
thermal shape of a plume for conditions as appropriate for
the Earth's mantle is cylindrical in an first order approximation
the shape of the entrained chemical material also has a very
strong sheet-like component. This can be important for the
understanding of the observed isotopic variability of plumes.
Back
http://earth.uni-muenster.de/~joergs
V51B-0526
Limitations
on the Estimation of Parental Magma Temperature Using Olivine-melt
Equilibria: Hotspots Not So Hot
* Natland, J H
(jnatland@rsmas.miami.edu) , RSMAS/MGG University of Miami,
4600 Rickenbacker Causeway, Miami, FL 33149 United States
Estimates of temperatures
of magmas parental to picritic tholeiites using olivine-melt
equilibria and FeO-MgO relationships depend strongly on the
assumption that a liquid composition, usually a glass, is
related to the most magnesian olivine in the rock, or to an
olivine composition in equilibrium with mantle peridotite,
along an olivine-controlled liquid line of descent. The liquid
Fe$^{2+}$/Fe$^{3+}$ also has to be known; where data exist,
average values from wet chemical determinations are used.
Crystallization histories of tholeiitic picrites from islands,
spreading ridges, and large igneous provinces, however, usually
reveal them to be hybrid rocks that are assembled by two types
of magma mixing: 1) between a) differentiated magmas that
are on olivine-plagioclase or olivine-plagioclase-clinopyroxene
cotectics and b) crystal sludges with abundant olivine that
may have accumulated from liquids crystallizing olivine alone;
and 2) between primitive magma strains in which olivine crystallized
either alone or with other silicate minerals at elevated pressure
on separate liquid lines of descent. Many picrites give evidence
that both types of mixing have occurred. If either type has
occurred, the assumption of olivine-control linking a glass
and an olivine composition can only circumstantially be correct.
Oxidation state can also be underestimated and therefore FeO
contents overestimated if basalts have degassed S, as at Hawaii.
In Case 1, hybrid host glass compositions often have higher
FeO at given MgO content than liquids which produced many
olivine crystals in the rock. In Case 2, the separate parental
melt strains are revealed by diversity of compositions of
both melt inclusions and Cr-spinel and are most often interpreted
to mean local heterogeneity of the mantle source. The inclusions
do not always affirm an olivine-controlled liquid line of
descent. Instead, inclusions with $<$13% Al$_{2}$O$_{3}$
are increasingly interpreted from both major oxides and trace
elements to be derived from melt strains produced by partial
melting of both depleted and enriched pyroxenite or recycled
ocean-crust (eclogite) (e.g., refs.1 and 2). Some Icelandic
picrites also contain large phenocrysts of plagioclase and
clinopyroxene; their abundant olivine evidently resulted from
mechanical processes of concentration of olivine such as flowage
differentiation. Using compositions of low-Al2O3 melt inclusions
and host liquids to estimate spinel compositions (ref. 3)
reveals many instances of crystallization at higher oxidation
states than occur during MORB crystallization, and successfully
predicts presence of spinel with Cr/(Cr+Al) = 60-75 actually
found in picrites from Hawaii, Iceland, elsewhere in the North
Atlantic Igneous Province, and the komatiites of Gorgona,
but not in MORB. Where fresh glass is lacking (e.g., Gorgona),
bulk-rock compositions have been used to reconstruct conditions
of crystallization of parental liquids; but this is greatly
complicated by the type and extent of alteration of the rocks.
The consequence of all of these factors is that FeO in presumed
olivine-controlled liquids is often overestimated, thus many
estimated temperatures of crystallization of primitive magnesian
liquids are too high by as much as 50-100$^{o}$ absolute,
and derived potential temperatures consequently are too high
by more than this. (1) Hansteen, T., 1991. Contrib. Mineral.
Petrol. 109, 225. (2) Sobolev, A., Hofmann, A., and Nikogosian,
I., 2000. Nature, 404, 986. (3) Poustovetov, A., and Roeder,
P., 2001, Canad. Mineral. 39, 309. Back
V51B-0527
Two-Stage Melting
Of Mantle Plumes And The Origin Of Rejuvenescent Volcanism
On Oceanic Islands
* White, W M (white@geology.cornell.edu)
, Cornell University, Dept. of Earth and Atmospheric Sciences,
Ithaca, NY 14853 United States
Morgan, J P (jpm@geology.cornell.edu) , Cornell University,
Dept. of Earth and Atmospheric Sciences, Ithaca, NY 14853
United States
Many mid-plate oceanic
volcanoes experience a rejuvenescent, or "post-erosional"
phase of volcanism that occurs hundreds of thousands or million
years after the main shield-building phase of volcanism has
ended. The Hawaiian Islands are the best-documented example,
but rejuvenescent volcanism also occurs on the Society Islands,
the Marquesas, the Australs, Samoa, and Mauritius. It does
not occur on near-ridge islands such as the Galapagos, the
Azores, and Iceland. Rejuvenescent lavas have a number of
features in common: they are erupted in small volumes, they
are highly enriched in incompatible elements, and they are
highly alkalic, typically basanitic to nephelenitic. All these
features suggest they are quite small degree melts. In addition,
rejuvenescent magmas have more depleted isotopic signatures,
implying they are melts of more depleted sources, despite
their strong incompatible element enrichment. Although isotopic
signatures of these lavas are more depleted that those of
the corresponding shield stage lavas, they are nevertheless
not as depleted as MORB. Furthermore, the isotopic compositions
of the rejuvenescent magmas rule out their sources being mixtures
of plume material and MORB-source material. Thus geochemical
considerations rule out both the lithosphere and the asthenosphere
surrounding the plume as the source of rejuvenescent magmas;
this implies the plume itself must be the source of rejuvenescent
magmas. This conclusion is consistent with geophysical models
of plumes. Finite difference numerical models of plume-lithosphere
interaction that include both temperature and compositional
viscosity dependence reveal that while most melting is concentrated
above the hot core of the plume, a melting "tail" extends
hundreds of km downstream. In this tail region, lateral spreading
of the plume results in a slight rising motion of the plume,
and consequently, small extents of melting. The problem thus
becomes that of deciphering why melts produced in this tail
region are isotopically distinct from those produced in the
main melting region. We propose the following model to explain
this difference: Mantle plumes are lithologically heterogeneous,
consisting of eclogite or pyroxenite "plums" that have a solidus
temperature several tens of degrees lower than the more refractory
peridotite "pudding" in which they are embedded. Complete
isotopic equilibrium is not achieved during melting - either
because the plums are large enough ($>$10-100m) or the
extraction of plum melts is rapid after their generation.
Both the plums and the peridotite are incompatible-element
enriched relative to the average depleted upper mantle, but
the plums are substantially more enriched. The plums melt
entirely in the base of the main melting region and the heat
so consumed initially suppresses melting of the peridotite
pudding. Plum-derived melts mix as they rise with melts of
the peridotite pudding produced higher in the main melting
region. This mixture of eclogitic and peridotitic melts form
the shield stage magmas. Material in the melting "tail" has
had the plums melted out of it in the main melting region.
Low degree melting of the plum-free peridotite in the melting
tail gives rise to rejuvenescent magmas. Melt production in
the tail is more or less continuous, but rejuvenescent volcanism
is not. This suggests that some other factor is involved,
such as lithospheric loading by adjacent volcanoes, that provides
pathways to the surface for small degree tail melts. Back
V51B-0528
How Many Plumes
In Africa ? The Geochemical Point of View
PIK, R (rpik@crpg.cnrs-nancy.fr)
, CRPG-CNRS, BP20, Vandoeuvre-Les-Nancy, 54501 France
* Marty, B (bmarty@crpg.cnrs-nancy.fr) , CRPG-CNRS, BP20,
Vandoeuvre-Les-Nancy, 54501 France
* Marty, B (bmarty@crpg.cnrs-nancy.fr) , ENSG, BP40, Vandoeuvre-Les-Nancy,
54501 France
Hilton, D (drhilton@ucsd.edu) , SCRIPPS, 8675 Discovery Way,
La Jolla, CA 92093 United States
Since the Oligocene,
volcanic activity in Africa was particularly important in
the Horn of Africa where ~ 1 million km3 of continental flood
basalts (the Ethiopian CFB) erupted 30 Ma ago in a time interval
of 1-2 Ma or less. The Afar volcanic province which is still
magmatically active is thought to represent the surface expression
of a deep mantle plume, a view consistent with ultra-low velocity
anomalies at the base of the mantle beneath the African superswell
and the Ethiopia-Afar volcanic province. This plume origin
is also supported by the occurrence of 3He/4He ratios up to
20 Ra (Ra is the 3He/4He ratio of atmospheric helium) much
higher than those of mid-ocean ridge basalts (on average,
8,b1 Ra) and thought to characterize mantle material originating
from below the 660 km discontinuity. However, a deep mantle
origin for "high 3He" material is currently questioned by
some models which rather ascribe a lithospheric or shallow
asthenospheric origin for such He component. The origin of
this signal can be tested with the distribution of He isotopic
signatures and other geochemical tracers among different African
volcanic provinces. All these other provinces exhibit 3He/4He
ratios that are equal to, or lower than, the mean MORB ratio
of 7-9 Ra (Cameroon line: 5-7 Ra; Hoggar: 8 Ra, this work;
Darfur 5.4-7.5 Ra; West African rift: 5-8.5 Ra, this work;
Comores, 6.5 Ra, this work). Although low 3He/4He ratios in
intraplate volcanic provinces could result from crustal recycling
in the mantle and remobilisation of recycled crust during
plume uprise, the upper range of 3He/4He values within the
field of MORB values points to the strong involvement of asthenospheric
mantle and limited interactions of magmas with the aged African
crust. Furthermore, these "low-3He" volcanic provinces are
characterized by strongly alkaline to undersaturated volcanism
indicative of low degrees of partial melting and a thermal
regime of the asthenosphere cooler than the one that gave
rise to transitional to tholeiitic Ethiopian CFBs. These geochemical
observations also conflict with models that advocate channelling
of the Afar hotspot material by pre-existing tectonic features
to account for all these African volcanic provinces. Back
V51B-0529
Recycling and
Mantle Stirring Determined by $^{142}$Nd/144Nd Isotopic Ratios
* Jacobsen, S
B (jacobsen@neodymium.harvard.edu) , Harvard University, Department
of Earth and Planetary Sciences, Cambridge, MA 02138 United
States
Ranen, M C (ranen@fas.harvard.edu) , Harvard University, Department
of Earth and Planetary Sciences, Cambridge, MA 02138 United
States
It is now well established
that $^{146}$Sm was live in the early solar system with an
initial uniform $^{146}$Sm/$^{144}$Sm ratio of ~0.008. Harper
and Jacobsen (1992) discovered that a sample from Isua (~3.8
Ga old) had a positive $^{142}$Nd/$^{144}$Nd anomaly of 33
ppm when compared to normal terrestrial and chondritic Nd.
Furthermore, Jacobsen and Harper (1996) reported results from
other Isua as well as Acasta (~4 Ga old) samples. Three other
Isua samples had a possible small range (about -15 to +15),
while two Acasta samples had no anomalies (normal to within
5 ppm). The presence of $^{142}$Nd anomalies at Isua has recently
been confirmed by two other groups (Boyet et al. 2003; Caro
et al. 2003). The available data demonstrate both the existence
of early depleted mantle and that the early mantle was isotopically
heterogeneous. As discussed by Jacobsen and Harper (1996),
the recycling rate can be determined by tracing the decay
of the average $^{142}$Nd/$^{144}$Nd value of the depleted
mantle. In addition, by using the $^{142}$Nd/$^{144}$Nd heterogeneity
in the depleted mantle through time we can determine the stirring
rate of the mantle (Kellogg, Jacobsen and O'Connell, 2002)
as a function of time. For this project our goal is to obtain
a resolution in $^{142}$Nd/$^{144}$Nd measurements of ~1 ppm.
We have thus compared results obtained for the Nd isotope
composition and $^{142}$Nd enriched standards for three different
TIMS instruments: The Finnigan MAT 262 at Harvard, the Isoprobe-T
and Finnigan TRITON mass spectrometers in GV Instrument's
and Thermo Electron's demo laboratories in Manchester and
Bremen, respectively. The Finnigan TRITON was designed in
response to a request from the senior author for such an instrument.
The results obtained so far demonstrate that all three instruments
yield the same $^{142}$Nd/$^{144}$Nd, $^{143}$Nd/$^{144}$Nd
and $^{145}$Nd/$^{144}$Nd isotopic ratios to within a few
ppm, while $^{148}$Nd/$^{144}$Nd and $^{150}$Nd/$^{144}$Nd
ratios agree to within 10-20 ppm, when all ratios are normalized
to $^{146}$Nd/$^{144}$Nd using the exponential law. Due to
the excellent agreement between results from three different
instruments we conclude that other reports that claimed that
such measurements could not be reproduced at the 5 ppm level
for either our 15 year old MAT262 or the newer instruments
must be in error. Acknowledgements: We thank GV Instruments
and Thermo Electron Corporation for making measurements of
our $^{142}$Nd enriched standards. Back
V51B-0530
The Importance
of Being Plumes: Entrainment, Isotopes, Melting and the Vertical
Structure of the Earth's Mantle
* DePaolo, D J
(depaolo@eps.berkeley.edu) , University of California, Dept.
of Earth and Planetary Science, Berkeley, CA 94720 United
States
Weaver, K L (karrie@eps.berkeley.edu) , University of California,
Dept. of Earth and Planetary Science, Berkeley, CA 94720 United
States
The standard approach
to hot spots has been to associate their geochemical characteristics
with the lower mantle. This, along with arguments about material
balance, has led to the notion that the composition of the
mantle beneath some depth horizon (e.g. 670 km) is different
from that above. This particular picture is difficult to reconcile
with ideas of whole mantle convection. Plumes, however, are
generally imagined as originating from a thermal/compositional
boundary layer, and probably at or near the base of the mantle.
Thus the fact that plume volcanic rocks are different from,
e.g. MORB, may mean only that the base of the mantle is distinct
from the rest of the mantle, and that there need be no major
compositional discontinuity suspended somewhere in the mid-mantle.
This point can be generalized - we should be able to get more
information from hot spots with better models and more systematic
sampling of lava flows. Theoretically, if plumes originate
near the bottom of the mantle, and then entrain some ambient
surrounding mantle on the way to the surface, the radial structure
of a plume should mimic the vertical structure of the mantle.
The full cross-section of the plume however, does not necessarily
pass through the melting zone, especially under thick lithosphere,
so the lavas represent only a fraction of the plume: the hottest
part. In areas where the lithosphere is thin, e.g. where plumes
impinge on very thin lithosphere, almost the full cross section
of the plume (and hence the full vertical section of the traversed
mantle) should be represented in the lavas. We use this concept
along with available geodynamic models to compare the observations
at Hawaii (thick lithosphere) with Iceland (thin or no lithosphere
along the ridge). In Hawaii the highest value of 87Sr/86Sr
is about 0.7037 near the plume axis (although it varies somewhat
with time). In Iceland the value is not much different at
0.7036. However, in Hawaii the lowest values in the shield
stage tholeiites are only slightly lower: 0.7035. In Iceland
the values extend continuously all the way down to the local
MORB values of 0.7027. In Hawaii, the width of the sampled
region is about 100 km, whereas in Iceland the width of the
sampled region along the ridge is about 600 km. There is consistency
between these patterns that is explained by the effect of
lithosphere thickness. Other relationships like this apply
for Nd, He, and Pb, and the length scales are not all the
same. The details of these distributions, if properly placed
into a context of plumes, entrainment and melting beneath
the lithosphere, could provide more information about the
vertical structure of the mantle, and detailed information
about the structure of the base of the mantle. A limiting
factor at present is geodynamic models to describe the plumes
and melting. Back
V51B-0531
MiFil: a method
to characterize seafloor swells with application to the South
Central Pacific
* Bonneville,
A (bonnevil@ipgp.jussieu.fr) , Institut de Physique du Globe
- CNRS, 4, place Jussieu, Paris, 75252 France
Adam, C (adam@ipgp.jussieu.fr) , Institut de Physique du Globe
- CNRS, 4, place Jussieu, Paris, 75252 France
Vidal, V (vidal@ipgp.jussieu.fr) , Institut de Physique du
Globe - CNRS, 4, place Jussieu, Paris, 75252 France
We propose a new filtering
method to characterize large-scale depth anomalies such as
seafloor swells associated to intraplate volcanism. Young
hotspot volcanoes within plate interiors are frequently surrounded
by smooth, broad regions of shallow seafloor termed midplate
swells. These swells are typically hundreds of kilometers
wide and can be more than a kilometer in elevation. The most
frequently invoked explanation for these swells is that they
represent the thermal and dynamic surface uplift from rising
mantle plumes but they can also be caused by underplating.
Wathever their origin be, these swells need to be precisely
characterized and we present here a simple method to do the
job. This method that we called MiFil, for Minimization and
Filtering, requires two stages: a first one to roughly remove
the volcano component by minimizing the depth anomaly; a second
one to smooth the shape and totally remove the small spatial
length scale remaining topography using a median filter. The
strength of this method, directly applicable on bathymetry
or seafloor depth anomaly grids is that it does not require
any assumption on the location, amplitude or width of the
large-scale feature to characterize, except its minimal width.
Application to hotspot volcanic chains of the South Central
Pacific is presented and the results lead to a better understanding
of the tectonics and volcanism emplacement of the zone. For
each chain, we determine the associated seafloor swell and
its main characteristics : (1) the Society is the only 'classical'
hotspot that corresponds to the simple interaction of a plume
with the lithosphere and for which a buoyancy flux of 1.58$\pm$0.15~Mg~s$^{-1}$
can be obtained; (2) the Marquesas volcanic chain, although
quite comparable, presents a swell morphology that prevents
such interpretation and quantification; (3) for the Tuamotu,
Pitcairn and Cook-Austral volcanic chains, no reliable quantification
can be made because the depth and geoid anomalies are caused
by several phenomena occurring at different depths that cannot
be separated. Back
V51B-0532
On the Zoology
of Mantle Upwellings : the case of the Pacific
Davaille, A (davaille@ipgp.jussieu.fr)
, Institut de Physique du Globe, 4 Place Jussieu, PARIS cedex
05, 75 252 France
* Bonneville, A (bonnevil@ipgp.jussieu.fr) , Institut de Physique
du Globe, 4 Place Jussieu, PARIS cedex 05, 75 252 France
Stutzmann, E (stutz@ipgp.jussieu.fr) , Institut de Physique
du Globe, 4 Place Jussieu, PARIS cedex 05, 75 252 France
The characteristics
of intraplate volcanism in the Pacific are very diverse. There
is probably more than $10^6$ seamounts on the Pacific seafloor
created by past or present volcanism. Among them, a number
are organized in alignments, some of which showing increasing
ages along the track in the direction of plate motion. Sorting
the latter by their track duration further shows two populations
: a big peak around 20 Myr and a few tracks longer than 50
Myr, sometimes originating at an oceanic basaltic plateau.
The Pacific presents also a number of isolated oceanic plateaus,
as well as the Polynesian superswell, a region of anomalously
shallow sea-floor several thousands of kms in extent with
an unusually dense concentration of alignments. Tomographic
images of the Pacific mantle reveal several 3D structures
of slow velocities, presumably indicating hot material. Moreover,
paleo-reconstructions show replication of volcanism over certain
areas on a 100 Myr time-scale. In a heterogeneous viscous
fluid like the mantle, several kinds of upwellings may develop,
from the classical, mushroom-shaped, thermal plume to more
complicated thermo-chemical structures. Fluid Mechanics studies
give definite constraints on the necessary conditions for
these upwellings existence, characteristics (spacing, recurrence
time, temperature anomaly,.), and ability to reach the lithosphere.
We therefore use those constraints to identify : 1) what types
of upwellings could develop in the Earth's mantle, 2) what
observations, if any, CAN be explained by a mantle upwelling,
3) what observations can NOT be explained by a mantle upwelling.
Back
V51B-0533
Large Igneous
Provinces, Mantle Plumes and Uplift: A Sedimentological perspective
Mazumder, R (mrajat2003@yahoo.com)
, Asutosh College, Dept. Geology, Kolkata, 700026 India
* Foulger, G R (g.r.foulger@durham.ac.uk) , University of
Durham, Science Laboratories, South Rd., Durham, Durham United
Kingdom
Significant pre-volcanic
uplift of the lithosphere is one of the expected consequences
of mantle plume upwelling. Many major mafic/ultramafic lavas
have been attributed to mantle plumes that are expected to
produce crustal uplift (doming) preceding the major phase
of volcanism. Such pre-volcanic uplift would have significant
consequences on the regional sedimentation pattern. Subsequent
erosion may remove much of the volcano-sedimentary record
of domal uplift. However, sedimentologists can identify progressive
shallowing of palaeogeography prior to volcanism and distinctive
palaeocurrent patterns if these exist, and this may provide
constraints on plume interpretations of the volcanic episode.
It is difficult to defend a LIP-mantle plume connection where
pre-volcanic lithospheric uplift is absent. This is particularly
true for continental flood basalts. The sedimentological and
stratigraphic criteria proposed for tracing LIP-plume connections
are somewhat generalized, and will not all be useful at a
given location. For example, progressive shallowing of palaeogeography
as a consequence of plume-induced lithospheric uplift should
be clear in a marine depositional setting (e.g., in the transition
from deep to shallow marine, or marine to terrestrial palaeogeography).
In a continental depositional setting, such evidence is less
likely to be found, however. This is because the depositional
surface is already well above mean sea level (the base level
of erosion) and its preservation potential is small. Unlike
the case of marine depositional settings, an erosional unconformity/sequence
boundary will thus not develop because the depositional surface
is already subaerially exposed. The claim that the sedimentary
record provides independent supporting evidence for mantle
plume influence on the generation of LIPs, is not always true.
The genetic linkage between CFBs and mantle plumes is at best
difficult to establish from sedimentological analysis alone.
Back
http://www.mantleplumes.org
V51B-0534
Heat Flow on
Hotspot Swells Reflect Fluid Flow Masking Potential Variations
in Mantle Heat Flux
* Harris, R N
(rnharris@mines.utah.edu) , University of Utah, Dept. of Geology
and Geophysics, 135 S 1460 E, Rm 719, Salt Lake City, UT 84112-0111
United States
McNutt, M K (marcia@mbari.org) , Monterey Bay Aquarium Research
Institute, 7700 Sandholdt Road, Moss Landing, CA 95039 United
States
The origin of hotspot
swells is poorly understood. Heat flow data collected on hotspot
swells have been used to argue for and against sublithospheric
thermal anomalies. The presence of sublithospheric thermal
anomalies has been inferred from interpretations of anomalously
high heat flow determinations, whereas the contention that
hotspot swells result from normal melting processes within
the lithosphere is based on `normal' heat flow values. These
arguments depend in part on the choice of a thermal reference
model, but more importantly assume conductive heat transfer
through the lithosphere. We provide evidence that heat flow
measurements collected on hotspot swells likely reflect shallow
fluid flow rather than deep thermal variations within or at
the base of the lithosphere. Discriminating between environments
where heat is transferred conductively or convectively requires
closely spaced heat flow determinations (1-2 km) collocated
with seismic reflection profiles. Only Hawaii and Reunion
have surveys meeting these requirements. The Hawaiian survey
consists of two profiles, one north of Oahu and one north
of Maro Reef. The Reunion survey also consists of two profiles,
both north of Mauritius. These heat flow profiles reveal greater
scatter than anticipated with spectral peaks on the order
of 10 km consistent with fluid flow. Root mean square variations
along the Oahu and Maro Reef profiles are 15 and 5 mW m$^{-2}$,
respectively, and along both Reunion profiles are about13
mW m$^{-2}$. Coupled heat and fluid flow models demonstrate
that thermal buoyancy due to bathymetric relief is capable
of driving significant fluid flow that may suppress the background
thermal field. These models are consistent with heat flow
patterns observed at individual seamounts and oceanic basement
highs that are more easily sampled and characterized than
large hotspot swells. We caution that the ability of fluid
flow to mask variations in sublithospheric heat flux make
surface heat-flow values a poor discriminator between geodynamic
models for hotspot swells. Back
V51B-0535
Cratonal lithosphere
a potential recorder of ancient mantle plumes
* Sleep, N H (norm@pangea.stanford.edu)
, Department of Geophysics, Stanford University, Stanford,
CA 94305 United States
In the conventional
mantle plume hypothesis, hot plume material ponds at the base
of the lithosphere. The deep cratonal lithosphere provides
a record and a potential test of this aspect of plumes. First
uplift and subsidence in the geological record give lthe more
likely times of plume impingement. Second studies of pressure-temperature-time
histories from zoned diamonds may make this process observable.
I make analytical and numerical calculations relevant the
the thermal history of the deep lithosphere. Physically stagnant-lid
convection subsequent to the ponding of plume material heats
the overlying lithosphere and cools the plume material. It
thins the rheological boundary layer of ordinary mantle between
overlying chemically buoyant cratonal lithosphere and the
underlying plume material. Convection moves the chemically
buoyant material producing a potentially observable pressure
change. Loitering plume tails have a stronger effect than
plume head episodes. They can occur only at times that the
plate moves slowly, which provides another potential test.
The xenolith geotherm provides a further constraints. If a
thick layer of plume material is present, the scaling relationship
for the thickness of the rheological boundary layer beneath
a convecting lid is inverse to the viscosity of the underlying
hot material to the fourth power at a given heat flow. Back
V51B-0536
Diagnostic
Features of Mantle Plume Penetration Into the Lithosphere.
* Jurine, D (jurine@ipgp.jussieu.fr)
, Institut de Physique du Globe de Paris, 4, place Jussieu,
Paris, 75005 France
Jaupart, C (cj@ccr.jussieu.fr) , Institut de Physique du Globe
de Paris, 4, place Jussieu, Paris, 75005 France
Brandeis, G (brandeis@ipgp.jussieu.fr) , Institut de Physique
du Globe de Paris, 4, place Jussieu, Paris, 75005 France
The success or failure
of the mantle plume hypothesis rests in part on its predictions
for the manner of plume penetration through the lithosphere.
We study how thermal plumes interact with viscous and buoyant
boundary layers which serve as analogs for oceanic and continental
lithospheres of various ages. Laboratory experiments and numerical
simulations have been performed over a large range of values
of plume buoyancy flux, lithosphere density, viscosity and
thickness. Plume penetration proceeds in two distinct phases.
In an initial phase, the lithosphere thins and is heated by
the plume. The extent of plume penetration depends on lithosphere
viscosity and plume buoyancy, and is not sensitive to plume
dimensions. In a second phase, heated lithosphere becomes
unstable and generates a secondary upwelling of larger dimensions
than the plume. The shape and dimensions of the intrusion
into the lithosphere vary significantly as a function of the
control parameters. Scaling laws have been derived for the
initial penetration velocity and for the critical time for
lithosphere instability. Application to the Earth leads to
a critical time equal to a few tens of My. The two penetration
phases are associated with different styles and rates of upwelling,
and hence different melting rates and magma compositions involving
mixtures of asthenospheric and lithospheric melts. Back
V51B-0537
Revised Models
of Global Plate Motions and Mantle Flow Successfully Predict
the Emperor-Hawaii and Other Hotspot-related Seamount Chains
* Sutherland,
R (r.sutherland@gns.cri.nz) , Institute of Geological and
Nuclear Sciences (GNS), PO Box 30-368, Lower Hutt, Wellington
New Zealand
Steinberger, B (bernhard.steinberger@ngu.no) , Institute for
Frontier Research on Earth Evolution (IFREE), Japan Marine
Science & Technology Center (JAMSTEC) 2-15 Natsushima-cho,
Yokosuka, 237-0061 Japan
O'Connell, R J (oconnell@geophysics.harvard.edu) , Department
of Earth and Planetary Sciences, Harvard University, 20 Oxford
Street, Cambridge, MA 02138 United States
The bend in the Hawaiian-Emperor
chain is a prominent feature usually attributed to a change
in Pacific plate motion. However, global plate motion chains
fail to predict that bend. We show how the geometry of the
Hawaiian-Emperor chain and other hotspot tracks can be explained
by a combination of hotspot motion and intraplate deformation.
Global mantle flow models predict a southward motion of the
Hawaiian hotspot. This, in combination with a plate motion
chain connecting Pacific and African plates through Antarctica,
predicts the Hawaiian track correctly since the bend, but
too far west prior to it. If a chain through Australia - Lord
Howe Rise is used instead, the track is predicted correctly
back to 65 Ma. The difference between the two predictions
indicates the effect of intraplate deformation not yet recognized
or else not recorded on the ocean floor. The remaining misfit
prior to 65 Ma can be attributed to additional intraplate
deformation of similar magnitude. Back
V51B-0538
Intermittent
Volcanism in the S Pacific: Tracking Persistent Geochemical
Sources
* Konter, J G
(jkonter@ucsd.edu) , SIO-UCSD, 9500 Gilman Dr, La Jolla, CA
92093-0225 United States
Koppers, A A (akoppers@ucsd.edu) , SIO-UCSD, 9500 Gilman Dr,
La Jolla, CA 92093-0225 United States
Staudigel, H (hstaudigel@ucsd.edu) , SIO-UCSD, 9500 Gilman
Dr, La Jolla, CA 92093-0225 United States
Hanan, B B (bhanan@sdsu.edu) , SDSU, 5500 Campanile Dr, San
Diego, CA 92182 United States
Blichert-Toft, J (jblicher@ens-lyon.fr) , ENS, 46 allee dItalie,
Lyon, 7 France
South Pacific ocean
intraplate volcanoes (OIV's) have formed relatively short,
discontinuous chains over the last 140 Ma, in contrast to
classic continuous hot spot chains like Hawaii or Louisville.
Moreover, its volcanism stands apart by very diverse and radiogenic
mantle source regions, defining the South Pacific Isotopic
and Thermal Anomaly (SOPITA). Discontinuous SOPITA OIV's form
a complex array of crossing lineaments and isolated seamounts
that can be back-tracked in time to the western Pacific. We
studied the Hf and Pb isotopic composition of the prominent
western Gilbert's and Tokelau chain that mostly correspond
to the plate motion stage expressed by the Emperor Seamounts.
The Gilbert's chain (to the West of Tokelau) has an age range
of 74-65 Ma, and back-tracks to an OIV source origin near
Rurutu. Notably, this chain shows an isotopic signature similar
to the Rurutu chain, from Burtaritari ($^{176}$Hf/$^{177}$Hf:
0.282900, $^{206}$Pb/$^{204}$Pb: 20.582) to Tuba (0.282938,
19.691). The parallel-running Tokelau chain ranges from 76-58
Ma, projecting current activity around Macdonald seamount.
Its isotopic composition is also similar to Macdonald seamount,
as shown by Howland (0.282951, 20.773) to Polo (0.283050,
19.390). However, both chains also contain seamounts with
ages and isotopic signatures that are outliers, inconsistent
with their general age range and isotopic trends (e.g. Ava;
0.282641, 17.729 and Sakau; 0.282880, 18.997). Our results
show that the discontinuous magmatic provinces within SOPITA
can be tracked back in time by geochronology and geochemical
fingerprinting. This suggests that the SOPITA OIV's are fed
by distinct, long-lived mantle source regions that may turn
"on" or "off" for distinct periods during their life-times.
This intermittent volcanism is inconsistent with the "standard"
fixed hot spot model, since it does not produce continuous
chains with linear age progressions. Furthermore, our results
demonstrate the utility of geochemical fingerprinting for
relating the products of discontinuous mantle melting anomalies.
Such combined evidence for long-lived but discontinuous OIV
lineaments allows for their use in plate and/or hot spot motion
models, and might offer a new understanding of the processes
responsible for the formation of OIV's. Back
V51B-0539
Improved Absolute
Plate Motion Modeling in the Pacific
* Wessel, P (pwessel@hawaii.edu)
, School of Ocean and Earth Science and Technology, University
of Hawaii at Manoa, 1680 East-West Rd, Honolulu, HI 96822
United States
Harada, Y (harada@scc.u-tokai.ac.jp) , School of Marine Science
and Technology, Tokai University, 3-20-1 Orido Shimizu, Shizuoka,
424-8610 Japan
Kroenke, L W (kroenke@soest.hawaii.edu) , School of Ocean
and Earth Science and Technology, University of Hawaii at
Manoa, 1680 East-West Rd, Honolulu, HI 96822 United States
In studies of Relative
Plate Motion (RPM), the model constraints are conjugate magnetic
isochrons identified in marine magnetic anomalies. The model
is a finite rotation that rotates an isochron on plate A such
that the rotated segment matches the conjugate isochron on
plate B. Chang (1987; 1988) solved for such rotations using
nonlinear spherical regression and developed statistical confidence
regions for the resulting rotations. Because conjugate data
can be optimally superimposed using a single, finite rotation
it was natural to define the model in terms of total reconstruction
rotations. In studies of Absolute Plate Motion (APM), the
constraints are the surface expressions of hotspot seamount
chains and their measured ages. The traditional approach is
to model coeval segments of seamount chains as small circles
about stage poles of rotation found by minimizing the distances
from each seamount to its locally best-fitting, small circle
about a candidate pole. The opening angles are typically found
by trial and error. Given the age range of a particular set
of copolar segments, opening rates can be determined. Because
the data portray small circles, it was natural to define the
model in terms of stage rotations. The traditional APM modelling
approach has many limitations, including (1) shorter segments,
possibly reflecting APM changes, are difficult to identify
and correlate across several chains; (2) short small-circle
segments become indistinguishable from great circles and hence
reliable poles cannot be determined; (3) without easily identifiable
kinks between chain segments, ages are needed to make the
correlation and these are often lacking; and (4) unlike RPM
modelling, no rigorous approach for estimating APM uncertainties
exists. However, Wessel and Kroenke (1997) developed a method
to derive optimal hotspot locations from seamount data if
the APM is known, whereas Harada and Hamano (2000) introduced
a technique to determine total reconstruction rotations if
hotspot locations are known. We improve the modelling of APM
by combining these two complimentary methods into a self-consistent
hybrid technique. The hybrid technique allows us to determine
(1) the best location for hotspots, (2) a high-resolution
APM model, and (3) covariance matrices for each rotation.
We present the first self-consistent Pacific APM with confidence
regions for each rotation pole and reconstructed points. The
new model is contrasted with traditional models, and the implications
of the model for drift within the Pacific hotspot group and
the origin of the Hawaii-Emperor bend is addressed. Back
V51B-0540
Paleomagnetic
Tests of Global Plate Reconstructions with Fixed and Moving
Hotspots
* Andrews, D L
(dandrews@jhu.edu) , Department of Earth Science, Rice University,
6100 Main St., Houston, TX 77005 United States
Gordon, R G (rgg@esci.rice.edu) , Department of Earth Science,
Rice University, 6100 Main St., Houston, TX 77005 United States
Horner-Johnson, B C (ben@esci.rice.edu) , Department of Earth
Science, Rice University, 6100 Main St., Houston, TX 77005
United States
Three distinct approaches
have been used in prior work to estimate the motion of the
Pacific basin plates relative to the surrounding continents.
The first approach is to use the global plate motion circuit
through Antarctica (e.g., the Pacific plate to the Antarctic
plate to the African plate to the North American plate). An
update to this approach is to incorporate the modest mid-Tertiary
motion between East and West Antarctica estimated by Cande
et al. (2000). A recently proposed second approach is to take
an alternative circuit for the early Tertiary of the Pacific
plate to the Australian plate to the East Antarctic plate
to the African plate to the North American plate (Steinberger
et al. 2004). The third approach is to assume that the hotspots
in the Pacific Ocean are fixed relative to those in the Atlantic
and Indian Oceans (e.g., Engebretson et al., 1986), which
we recently showed indicates motion between East and West
Antarctica of 800 $\pm$ 500 km near the Ross Sea Embayment.
The first approach (global plate motion circuit through Antarctica)
indicates very rapid motion between Pacific and Indo-Atlantic
hotspots during the early Tertiary (e.g., Raymond et al. 2000).
The second approach (global plate motion circuit through Australia)
indicates slower, but still substantial, motion between Pacific
and Indo-Atlantic hotspots (Steinberger et al. 2004). Because
each of the three approaches predicts distinctly different
motion between the Pacific plate and the continental plates,
they can be tested with paleomagnetic data. The results of
such tests indicate that the first approach leads to systematic
and significant misfits between Pacific and non-Pacific early
Tertiary and Late Cretaceous paleomagnetic poles. The second
approach leads to slightly smaller misfits. In contrast, the
circuit based on fixed hotspots brings the Pacific and non-Pacific
paleomagnetic poles into consistency. Thus the paleomagnetic
data decisively favor fixed hotspots over the alternative
approaches and suggests that motion between hotspots is substantially
less than inferred by Steinberger et al. (2004). Back
V51B-0541
Latitudinal
Shift of the Hawaiian Hotspot: Motion Relative to Other Hotspots
or True Polar Wander?
* Gordon, R G
(rgg@rice.edu) , Rice University, 6100 Main St. Earth Science--MS
126, Houston, TX 77005 United States
Horner-Johnson, B C (ben@esci.rice.edu) , Rice University,
6100 Main St. Earth Science--MS 126, Houston, TX 77005 United
States
Recent results from
deep sea drilling confirm a large southward drift of the Hawaiian
hotspot since Campanian and Maastrichtian time (ca. 70 to
83 Ma), as was previously found from prior paleomagnetic results
from drilling (Kono, 1980; Jackson et al. 1980), from skewness
analysis of Pacific magnetic anomalies (Gordon 1982, Petronotis
& Gordon 1989, 1999; Petronotis et al. 1992, 1994; Acton
& Gordon, 1991; Vasas et al. 1994; Horner-Johnson &
Gordon 2003), and from other paleomagnetic and paleolatitude
data (Gordon & Cape 1981; Sager & Bleil 1987). This
southward drift could have been the result of motion of the
Hawaiian hotspot relative to some other hotspots, or of true
polar wander, or of both. Tarduno et al. (2003) have recently
presented an extreme interpretation of these results as being
entirely due to southward motion of the Hawaiian hotspot through
the mantle. Here we show that this extreme interpretation
is not supported by available data. While the Pacific plate
paleomagnetic data are sufficient to show that the Hawaiian
hotspot has moved southward relative to the spin axis, alone
they cannot be used to demonstrate motion relative to the
mantle or relative to other hotspots. To do so, coeval paleomagnetic
poles are needed from the continents bordering the Atlantic
and Indian Oceans. Here we show that few, if any, of the coeval
paleomagnetic poles from the continents incorporated into
widely used reference paths pass minimum reliability criteria.
Thus, the inference of rapid motion of the Hawaiian hotspot
relative to the mantle is surely premature and probably incorrect.
We further show that other earlier studies purporting to show
motion between hotspots from paleomagnetic data are now invalid
because of revisions to paleomagnetic poles from the continents
or because of flaws in analysis. Updated paleomagnetic analyses
indicate that little motion has occurred between Pacific hotspots
and non-Pacific hotspots. Instead, available data are consistent
with the hypothesis that the southward motion of the Hawaiian
hotspot relative to the spin axis is mainly caused by true
polar wander. Back
V51B-0542
Fifty Million
Years of Fixed Hotspots: A New Self-Consistent Global Set
of Plate Rotations
* Kumar, R R (rkumar@rice.edu)
, Rice University, 6100 Main St. Earth Science--MS 126, Houston,
TX 77005 United States
Andrews, D L (dandrews@jhu.edu) , Rice University, 6100 Main
St. Earth Science--MS 126, Houston, TX 77005 United States
Gordon, R G (rgg@rice.edu) , Rice University, 6100 Main St.
Earth Science--MS 126, Houston, TX 77005 United States
We have developed new
methods for appropriately estimating the uncertainties in
plate rotations relative to hotspots. Using these new methods
and the latest available relative plate rotations, and also
incorporating uncertainties in relative plate motions, we
showed that there is no significant motion between Pacific
hotspots and Indo-Atlantic hotspots for the past ca. 50 Myr.
Here we take the next step and seek a sequence of rotations
based on simultaneous inversion of hotspot track data in the
Pacific, Atlantic, and Indian Oceans. The resulting set of
reconstructions are thus optimal estimates for global plate
motion relative to the hotspots for the past 50 Myr. Evidence
for significant changes in plate motion, including changes
in pole of rotation and in rates of rotation, will be presented.
Implications for true polar wander will be discussed. Back
V51B-0543
Is the Hawaiian-Emperor
Bend Coeval for all Pacific Seamount Trails?
* Koppers, A A
(akoppers@ucsd.edu) , IGPP, Scripps Institution of Oceanography,
University of California, San Diego, La Jolla, CA 92093-0225
United States
Staudigel, H (hstaudig@ucsd.edu) , IGPP, Scripps Institution
of Oceanography, University of California, San Diego, La Jolla,
CA 92093-0225 United States
By far the largest
number of hotspots can be found in the South Pacific Thermal
and Isotopic Anomaly (SOPITA). Its Cretaceous counterpart
is preserved in a large range of seamounts and guyots found
in the West Pacific Seamount Province (WPSP). The seamounts
in these regions display very distinct and long-lived isotopic
signatures (Staudigel et al., 1991; Koppers et al., 2003)
that can be used to combine source region chemistry and seamount
geochronology to map out mantle melting anomalies over geological
time. These mappings may resolve many important questions
regarding the stationary character, continuity and longevity
of the hotspots in the South Pacific mantle. Most importantly,
it may also answer the question whether the Hawaiian-Emperor
Bend (HEB) is coeval for all Pacific Seamount trails at 47
Ma? Fixed hotspots should be expressed in volcanic trails
on the lithospheric plates revealing absolute rates of motion
from their age progressions and the direction of motion based
on their azimuths. By definition, bends in these hotspot trails
thus should give an indication of changing plate motion happening
simultaneously across individual lithospheric plates. Based
on the morphology of seamounts in the Pacific, the Hawaiian-Emperor,
Louisville, Gilbert Ridge and Tokelau seamount trails may
be identified as the only hotspot trails to exhibit a clear
HEB-type bend (Kroenke et al. 2004). Of these, the Louisville
seamount trail only displays a faint bend that may be coeval
with the sharp 60 degree bend in the Hawaiian-Emperor trail
(Koppers et al. 2004) at 47 Ma. However, new 40Ar/39Ar analyses
indicate that the HEB-type bends in the Gilberts Ridge and
Tokelau seamount trails are asynchronous around 67 Ma and
57 Ma, respectively. We argue, therefore, that plate motion
alone cannot explain these age systematics, but that both
hotspot motion and changing lithospheric stress regimes may
play an important role in their creation. The simple and elegant
hotspot model that (almost without difficulty) may explain
primary hotspots such as Hawaii and Louisville, seems unsatisfactory
to explain the age distributions of the short-lived Gilbert
Ridge and Tokelau hotspots. To explain intra-plate volcanism
in the South Pacific, we argue for a combination of processes:
one that forces regional magmatism from a large-scale source
of buoyancy from below (like the rise of plumelets shooting
off the top of a superplume that die-off after a short life-cycle)
and one process that acts from above, as intra-plate extension
opens up pathways that allow the lithosphere to be penetrated
by magma. Back
http://earthref.org/databases/SC/
V51B-0544
Critical Evaluation
of Radiometric Ages Used for Tracking Hotspots in the Pacific
Ocean
* Baksi, A K (abaksi@geol.lsu.edu)
, Department of Geology, Louisiana State University, Baton
Rouge, LA 70803 United States
One of the pillars
supporting the plume hypothesis, is the progression of ages
for numerous hotspot tracks in oceans. These ages should be
based on radiometric measurements. The argon dating methods
have been the tool most commonly used. Since most of the rocks
selected for dating have suffered (considerable) alteration,
K-Ar dates should not be used as accurate measures of the
age of crystallization. 40Ar/39Ar total fusion ages, though
better than K-Ar dates in general, do not pinpoint samples
that (a) contain excess argon or (b) have suffered partial
loss of 40Ar* due to alteration. Hence 40Ar/39Ar incremental
heating studies remain as the (only) tool of choice. From
such experiments, at a minimum, ages must be based on plateau
and/or isochron sections that meet the necessary statistical
requirements to be considered crystallization ages. Earlier
(Baksi, 1999, Jour. Geol.) it has been shown that almost all
the purported crystallization ages for hotspot tracks in the
Atlantic and Indian Oceans, are invalid (see also www.mantleplumes.org/ArAr.html).
Herein, I apply the tests outlined therein, to evaluate ages
available in the literature for hotspot tracks in the Pacific
Ocean. These can be divided into five main groups. (1) Those
with reliable age data (e.g. Dalrymple and Garcia,1980; Dalrymple
et al., 1980, DSDP 55, Hawaiian-Emperor Chain); the authors
use care in selecting valid ages from their data sets. (2)
Others (e.g. Pringle, 1993, AGU Monograph 77, Musicians Seamounts),
most ages are statistically valid, though some fail the requisite
test. In addition, many samples show high levels of atmospheric
argon, suggesting the samples are (quite) altered; this could
lead to incorrect plateau ages. (3) The next set (e.g. Winterer
et al., 1993, AGU Monograph 77, Cretaceous guyots in the Northwest
Pacific; Ozima et al., 1977, JGRAS, Western Pacific guyots;
Saito and Ozima, 1977, EPSL, Western Pacific area) have few,
if any, valid ages. Most plateaux/isochrons clearly fail the
statistical test of reliability; many steps show high levels
of atmospheric argon - the samples are (badly) altered. (4)
A set of papers (e.g. Gripp and Gordon, 2002, Geophys. J.
Int., young hotspot tracks; Duncan, 1985 - New Hebrides-Samoa
lineament) make use of K-Ar dates, wholly or in the main.
These dates should be treated as minimum estimates of the
crystallization age. (5) A final set of papers (Sager et al.,
1993, AGU Monograph 77, Japanese and Marcus-Wake Seamounts;
Lincoln et al., 1993, AGU Monograph 77, Marshall Islands),
quote ages without listing the relevant analytical data. These
results are to be treated as suspect, and not used for quantitative
tracking of hotspot trails. In conclusion, the purported progression
of ages for numerous hotspot tracks in the Pacific Ocean does
not withstand critical scrutiny. Back
V51B-0545
40Ar/39Ar Geochronology
of the Sylhet Traps, Eastern India, and their relationship
to the Kerguelen Plume related magmatism
* Ray, J S (jsray@prl.ernet.in)
, Physical Research Laboratory, Navrangpura, Ahmedabad, 380009
India
Pande, K (kanchanpande@iitb.ac.in) , Physical Research Laboratory,
Navrangpura, Ahmedabad, 380009 India
It is now an accepted
view that the Rajmahal-Bengal flood basalts of eastern India
belong to the Kerguelen plume generated Large Igneous Province
(LIP) that encompasses the Bunbury-Naturaliste Plateau basalts
of western Australia, volcanism on the southern and central
Kerguelen Plateau, Elan Blank, and Broken Ridge in the Indian
Ocean (e.g., Kent et al., J. Petrol., 43, 2002). The lesser-known
Sylhet Traps (25.5$^{o}$ N, 91.8$^{o}$ E), exposed ${\sim}$400
km east of the Rajmahal Traps, are assumed to be a part of
the ${\sim}$118 Ma old Rajmahal-Bengal flood basalt province
(e.g., Bakshi, Chem. Geol., 121, 1995). However, absence of
physical continuity between these volcanic activities, and
more importantly lack of precise age data always created difficulty
in correlations. In an attempt to test the hypothesis of their
cogenesis we dated the Sylhet Traps using $^{40}$Ar/$^{39}$Ar
incremental-heating technique. Two whole rock samples yielded
good plateau ages: 115.9{$\pm$}4.1 (2{$\sigma$}) Ma, and 115.5{$\pm$}5.4
(2{$\sigma$}) Ma, calibrated against an age of 523.2{$\pm$}0.9
Ma for the Minnesota Hornblende (MMhb-1; Spell and McDougall,
Chem. Geol., 198, 2003). The concordant plateau, isochron
and inverse isochron ages, the atmospheric value for the trapped
$^{40}$Ar/$^{36}$Ar component, and good MSWD values for the
isochrons suggest that 115.8{$\pm$}3.2 Ma (the weighted mean
of both the ages) can be considered as the age of crystallization.
This age for the Sylhet Traps clearly falls within the range
of ages reported for various groups of lavas in the Rajmahal-Bengal
province (118-115), and is consistent with the idea that magmatism
in this part of India continued well beyond the major pulse
at ${\sim}$118 Ma (e.g., Kent et al., J. Petrol., 43, 2002).
It also supports the proposal based on geochronology and geochemistry
that the Kerguelen hotspot was located close to the eastern
Indian margin during its peak activity (e.g., Kumar et al.,
Geophys. Res. Lett., 30, 2003). Back
V51B-0546
High precision
Pb, Sr, and Nd isotope geochemistry of alkalic early Kilauea
magmas from the submarine Hilina bench region, and the nature
of the Hilina/Kea mantle component
* Kimura, J (jkimura@riko.shimane-u.ac.jp)
, Shimane University, Nishikawatsu 1060, Matsue, 690-8504
Japan
Sisson, T W (tsisson@usgs.gov) , US Geological Survey, Middlefield
Road, Menlo Park, CA 94025 United States
Nakano, N (s029207@matsu.shimane-u.ac.jp) , Shimane University,
Nishikawatsu 1060, Matsue, 690-8504 Japan
Coombs, M L (mcoombs@usgs.gov) , US Geological Survey, Middlefield
Road, Menlo Park, CA 94025 United States
Lipman, P W (plipman@usgs.gov) , US Geological Survey, Middlefield
Road, Menlo Park, CA 94025 United States
Submarine lavas recovered
from the Hilina bench region, offshore Kilauea, Hawaii Island
provide information on ancient Kilauea volcano and the geochemical
components of the Hawaiian hotspot. Alkalic lavas, including
nephelinite, basanite, hawaiite, and alkali basalt, dominate
the earliest stage of Kilauea magmatism. Transitional basalt
pillow lavas are an intermediate phase, preceding development
of the voluminous tholeiitic subaerial shield and submarine
Puna Ridge. Most alkalic through transitional lavas are quite
uniform in Sr-Nd-Pb isotopes, supporting the interpretation
that variable extent partial melting of a relatively homogeneous
source was responsible for much of the geochemical diversity
of early Kilauea magmas (Sisson et al., 2002). These samples
are among the highest 206Pb/204Pb known from the Hawaii islands
and may represent melts from a distinct geochemical and isotopic
endmember involved in the generation of most Hawaiian tholeiites.
This endmember is similar to the postulated literature Kea
component, but we propose it should be renamed Hilina, to
avoid confusion with the geographically defined Kea-trend
volcanoes. Isotopic compositions of some shield-stage Kilauea
tholeiites overlap the Hilina endmember but most deviate far
into the interior of the isotopic field defined by magmas
from other Hawaiian volcanoes, reflecting the introduction
of melt contributions from both _gKoolau_h (high 87Sr/86Sr,
low 206Pb/204Pb) and depleted (low 87Sr/86Sr, intermediate
206Pb/204Pb) source materials. This shift in isotopic character
from nearly uniform, endmember, and alkalic, to diverse and
tholeiitic corresponds with the major increase in Kilauea_fs
magmatic productivity. Two popular geodynamic models can account
for these relations: (1) The upwelling mantle source could
be concentrically zoned in both chemical/isotopic composition,
and in speed/extent of upwelling, with Hilina (and Loihi)
components situated in the weakly ascending margins and the
Koolau component in the interior. The depleted component could
be refractory and spread throughout or scavenged from the
overlying lithosphere. (2) The Hilina (and Loihi) components
could be more fertile material (lower melting temperature)
spread irregularly throughout the Hawaiian source in a matrix
of more refractory depleted and Koolau compositions. Modest
upwelling along the leading hotspot margin melts the fertile
domains predominantly, while the refractory matrix also partially
melts in the more vigorously upwelling hotspot interior, diluting
the Hilina and Loihi components and yielding voluminous isotopically
diverse tholeiitic magmas. Back
V51B-0547
Submarine Alkalic
Lavas Around the Hawaiian Hotspot; Plume and Non-Plume Signatures
Determined by Noble Gases
* Hanyu, T (hanyut@jamstec.go.jp)
, Institute for Frontier Research on Earth Evolution, Japan
Agency for Marine-Earth Science and Technology, Natsushima-cho
2-15, Yokosuka, 237-0061 Japan
Clague, D A (clague@mbari.org) , Monterey Bay Aquarium Research
Institute, 7700 Sandholdt Road, Moss Landing, CA 95039 United
States
Kaneoka, I (Ikaneoka@aol.com) , Earthquake Research Institute,
University of Tokyo, Yayoi 1-1-1, Bunkyo-ku, Tokyo, 113-0032
Japan
Dunai, T J (dunt@geo.vu.nl) , Faculty of Earth and Life Sciences,
Vrije Universiteit Amsterdam, De Boelelaan 1085, Amsterdam,
1081HV Netherlands
Davies, G R (gareth.davies@falw.vu.nl) , Faculty of Earth
and Life Sciences, Vrije Universiteit Amsterdam, De Boelelaan
1085, Amsterdam, 1081HV Netherlands
Noble gas isotopic
ratios were determined for submarine alkalic volcanic rocks
distributed around the Hawaiian islands to constrain the origin
of such alkalic volcanism. Samples were collected by dredging
or using submersibles from the Kauai Channel between Oahu
and Kauai, north of Molokai, northwest of Niihau, Southwest
Oahu, South Arch and North Arch volcanic fields. Sites located
downstream from the center of the hotspot have 3He/4He ratios
close to MORB at about 8 Ra, demonstrating that the magmas
erupted at these sites had minimum contribution of volatiles
from a mantle plume. In contrast, the South Arch, located
upstream of the hotspot on the Hawaiian Arch, has 3He/4He
ratios between 17 and 21 Ra, indicating a strong plume influence.
Differences in noble gas isotopic characteristics between
alkalic volcanism downstream and upstream of the hotspot imply
that upstream volcanism contains incipient melts from an upwelling
mantle plume, having primitive 3He/4He. In combination with
lithophile element isotopic data, we conclude that the most
likely source of the upstream magmatism is depleted asthenospheric
mantle that has been metasomatised by incipient melt from
a mantle plume. After major melt extraction from the mantle
plume during production of magmas for the shield stage, the
plume material is highly depleted in noble gases and moderately
depleted in lithophile elements. Partial melting of the depleted
mantle impregnated by melts derived from this volatile depleted
plume source may explain the isotopic characteristics of the
downstream alkalic magmatism. Back
V51B-0548
A Combined
He and Os Isotopic Study of the HSDP-2 Core from Mauna Kea,
Hawaii
* Ireland, T J
(tireland@geol.umd.edu) , Department of Geology, University
of Maryland, College Park, MD 20742 United States
Walker, R J (rjwalker@geol.umd.edu) , Department of Geology,
University of Maryland, College Park, MD 20742 United States
DePaolo, D J (depaolo@eps.berkeley.edu) , Department of Earth
and Planetary Science, University of California, Berkeley,
CA 94720-4767 United States
Kurz, M D (mkurz@whoi.edu) , Department of Marine Chemistry
and Geochemistry, Woods Hole Oceanographic Institution, Woods
Hole, MA 02543 United States
Combined osmium and
helium isotope systematics of hotspot lavas have the potential
to reveal information about the deep Earth. A high $^{3}$He/$^{4}$He
ratio may represent an undegassed reservoir, generally associated
with the lower mantle. There are two Os isotopes that can
be studied to help to further elucidate the problem. The decay
of $^{187}$Re to $^{187}$Os is the more frequently cited system;
however, in terms of lower mantle processes, the decay of
$^{190}$Pt to $^{186}$Os may be extremely useful. Both of
these Os isotopes are enriched in the core relative to chondritic
values. In a previous study, Brandon {\it et al}. (1999) examined
several Hawaiian volcanoes for both He and Os isotopes. A
correlation was noted between the $^{3}$He/$^{4}$He, $^{187}$Os/$^{188}$Os
and $^{186}$Os/$^{188}$Os ratios. In terms of $^{3}$He/$^{4}$He
and $^{187}$Os/$^{188}$Os space, the three commonly cited
Hawaiian end-members (Kea, Koolau and Loihi members) were
clearly defined. A strong positive correlation was also observed
for $^{186}$Os/$^{188}$Os versus $^{3}$He/$^{4}$He. These
correlations were interpreted as a possible signature of core-mantle
interaction. There were some limitations to previous studies.
Only 2-3 samples from each volcano were studied, with these
samples generally being subaerially erupted. The He data utilized
were often not for the same samples for which the Os data
were collected (volcano averages for He were used on some
samples). With the introduction of data from the Hawaiian
Scientific Drilling Project (HSDP-2), which drilled 2.84 km
into the Mauna Kea volcanics (DePaolo {\it et al}., 2000),
an extensive history of a single volcano can be observed (from
the early submarine stages to the later subaerial rocks).
In the current study a detailed Os isotopic analysis of several
samples that span a large depth range of the HSDP-2 core,
in conjunction with previously collected He isotopic data
(Kurz {\it et al}., 2004), was conducted. The samples define
a relatively narrow range of slightly suprachondritic $^{187}$Os/$^{188}$Os
ratios (0.12865-0.13056), despite having a large spread in
$^{3}$He/$^{4}$He (8.5-23.5 Ra). This result indicates that
the $^{187}$Os/$^{188}$Os ratios for Mauna Kea may reflect
contributions from a relatively constant source in terms of
Os, while the He characteristics of the source appear to be
highly variable. Anticipated $^{186}$Os measurements will
likely add additional insight to the causes of the isotopic
heterogeneities. Back
V51B-0549
The Chemical
Structure of the Hawaiian Mantle Plume
* Ren, Z (ren@geo.titech.ac.jp)
, Earth and Planetary Sciences, Tokyo Institute of Technology,
2-12-1 Ookayama, Meguroku, Tokyo, 152-8551 Japan
Hirano, N (nhirano@geo.titech.ac.jp) , Earth and Planetary
Sciences, Tokyo Institute of Technology, 2-12-1 Ookayama,
Meguroku, Tokyo, 152-8551 Japan
Hirata, T (hrt1@geo.titech.ac.jp) , Earth and Planetary Sciences,
Tokyo Institute of Technology, 2-12-1 Ookayama, Meguroku,
Tokyo, 152-8551 Japan
Takahashi, E (etakahas@geo.titech.ac.jp) , Earth and Planetary
Sciences, Tokyo Institute of Technology, 2-12-1 Ookayama,
Meguroku, Tokyo, 152-8551 Japan
Ingle, S (single@geo.titech.ac.jp) , Earth and Planetary Sciences,
Tokyo Institute of Technology, 2-12-1 Ookayama, Meguroku,
Tokyo, 152-8551 Japan
Numerous geochemical
studies of Hawaiian basaltic lavas have shown that the Hawaiian
mantle plume is isotopically heterogeneous. However, the distribution
and scale of these heterogeneities remain unknown. This is
essentially due to the complex interactions created by melting
a heterogeneous source, subsequent aggregation of the melts
on their way to the surface, and mixing that takes place in
shallow magma chambers prior to eruption. In sum, the measured
compositions of bulk lavas may represent only _eaverage_f
compositions that do not fully reflect the complexity of either
the mantle source heterogeneity and/or chemical structure.
Melt inclusions, or samples of the local magma frozen in olivine
phenocrysts during their formation, are better at recording
the complex magmatic history than are the bulk samples. Here,
we report major and trace element compositions of olivine-hosted
melt inclusions from submarine Haleakala lavas that were collected
by 2001-2002 JAMSTEC cruises measured by EPMA and LA-ICP-MS
after homogenization at $1250\deg$C, QFM for 20min. Melt inclusions
from the submarine Hana Ridge (Haleakala volcano) show large
ranges in CaO/Al$_{2}$O$_{3}$ (0.92-1.50), TiO$_{2}$/Na$_{2}$O
(0.79-1.60) and Sr/Nb (14.56-36.60), Zr/Nb (6.48-16.95), ranging
from Kilauea-like to Mauna Loa-like compositions within separately-sampled
lavas as well as in a single host lava sample. Bulk rocks
geochemistry shows that major element composition and trace
element ratios such as Zr/Nb, Sr/Nb (Ren et al., 2004a, in
press, J. Petrol.) together with Pb, Nd and Sr isotopic ratios
(Ren et al., 2004b, submitted to J. Petrol.) of Haleakala
shield volcano also display systematic compositional variation
changing from a Kilauea-like in the submarine Hana Ridge (main
shield stage) to Kilauea-Mauna Loa-like in the subaerial Honomanu
stage (late shield stage, data from Chen and Frey, 1991).
Some of the compositional variations in melt inclusions in
single rocks are wider range than over-all variation observed
in bulk rocks. It is important that both Kilauea-like and
Mauna Loa-like compositions co-exist in melt inclusions in
single submarine Hana Ridge rocks which are identified as
Kilauea-like based on bulk geochemistry. These observations
are inconsistent with the current interpretation that magma
compositions are controlled by concentric zonation of the
Hawaiian mantle plume (e.g. Kea component and Loa component),
manifested as the Kea trend and the Loa trend volcanoes (e.g.
Hauri, 1996; Lassiter et al., 1996). Our new data from olivine-hosted
melt inclusions imply that the chemical structure of the Hawaiian
mantle plume is significantly more complicated than previously
modeled and the length-scale of chemical heterogeneity must
be remarkably smaller than estimated based on bulk rock geochemistry.
Back
V51B-0550
Extreme Hf-Os
Isotope Compositions in Hawaiian Peridotite Xenoliths: Evidence
for an Ancient Recycled Lithosphere
* Bizimis, M (bizimis@magnet.fsu.edu)
, Dept. Earth Sciences, FIU, 11200 SW 8th Street,, Miami,
FL 33199 United States
* Bizimis, M (bizimis@magnet.fsu.edu) , NHMFL and Dept. Geological
Sciences, FSU, 1800, E. Paul Dirac, Dr., Tallahassee, FL 32306
United States
Lassiter, J C (lassiter1@mail.utexas.edu) , Max Plank Institute
f. Chemie, Postfach 3060, Mainz, 55020 Germany
Salters, V J (salters@magnet.fsu.edu) , NHMFL and Dept. Geological
Sciences, FSU, 1800, E. Paul Dirac, Dr., Tallahassee, FL 32306
United States
Sen, G (seng@fiu.edu) , Dept. Earth Sciences, FIU, 11200 SW
8th Street,, Miami, FL 33199 United States
Griselin, M (griselin@mpch-mainz.mpg.de) , Max Plank Institute
f. Chemie, Postfach 3060, Mainz, 55020 Germany
We report on the first
combined Hf-Os isotope systematics of spinel peridotite xenoliths
from the Salt Lake Crater (SLC), Pali and Kaau (PK) vents
from the island of Oahu, Hawaii. These peridotites are thought
to represent the Pacific oceanic lithosphere beneath Oahu,
as residues of MORB-type melting at a paleo-ridge some 80-100Ma
ago. Clinopyroxene mineral separates in these peridotites
have very similar Nd and Sr isotope compositions with the
post erosional Honolulu Volcanics (HV) lavas that bring these
xenoliths to the surface. This and their relatively elevated
Na and LREE contents suggest that these peridotites are not
simple residues of MORB-type melting but have experience some
metasomatic enrichment by the host HV lavas. However, the
SLC and PK xenoliths show an extreme range in Hf isotope compositions
towards highly radiogenic values ($\epsilon$$_{Hf}$= 7-80),
at nearly constant Nd isotope compositions ($\epsilon$$_{Nd}$=
7-10), unlike any OIB or MORB basalt. Furthermore, these Oahu
peridotites show a bimodal distribution in their bulk rock
$^{187}$Os/$^{186}$Os ratios: the PK peridotites have similar
ratios to the abyssal peridotites (0.130-0.1238), while the
SLC peridotites have highly subchondritic ratios (0.1237-0.1134)
that yield 500Ma to 2Ga Re-depletion ages. Hf-Os isotopes
show a broad negative correlation whereby the samples with
the most radiogenic $^{176}$Hf/$^{177}$Hf have the most unradiogenic
$^{187}$Os/$^{186}$Os ratios. Based on their combined Hf-Os-Nd
isotope and major element compositions, the PK peridotites
can be interpreted as fragments of the Hawaiian lithosphere,
residue of MORB melting 80-100Ma ago, that have been variably
metasomatized by the host HV lavas. In contrast, the extreme
Hf-Os isotope compositions of the SLC peridotites suggest
that they cannot be the source nor residue of any kind of
Hawaiian lavas, and that Hf and Os isotopes survived the metasomatism
or melt-rock reaction that has overprinted the Nd and Sr isotope
compositions of these peridotites. The ancient ($>$1Ga)
melt depletion event recorded by both the low $^{187}$Os/$^{186}$Os
and high $^{176}$Hf/$^{177}$Hf ratios in the SLC peridotites
can be explained with two different scenarios. First, the
SLC peridotites may represent ancient depleted lithosphere
that survived subduction, remained "rafting" in the upper
mantle and is now sampled beneath Oahu. However, the lack
of such unradiogenic Os isotopes in both MORBs and abyssal
peridotites suggests that such peridotites are rare in the
upper mantle and makes their exclusive presence under Oahu
a rather fortuitous coincidence. Alternatively, the SLC peridotites
may represent ancient depleted recycled lithosphere brought
up by the Hawaiian plume. A recycled oceanic crust origin
has been previously invoked for the Koolau shield lavas. It
is then conceivable that fragments of the lithospheric portion
of that subducted package have remained coupled with the oceanic
crust and are being brought up by the plume from the deep,
but because they were previously depleted, these peridotites
contribute minimally, if at all, to Hawaiian volcanism. The
presence of microdiamonds and majoritic garnets in some SLC
pyroxenites also corroborates a deep origin. In this case,
the SLC peridotites represent the first-ever direct evidence
that subducted material actually makes it back on the surface,
essentially closing the subduction cycle. Back
V51B-0551
He, Sr, Nd,
and U Isotopic Variations in Post-Shield Lavas From the Big
Island of Hawaii - Insight Into Magma Transport and the Chemical
Structure of the Hawaiian Plume.
* Aciego, S M
(aciego@eps.berkeley.edu) , Department of Earth and Planetary
Science, University of California, Berkeley, CA 94720-4767
* Aciego, S M (aciego@eps.berkeley.edu) , Earth Science Division,
E.O. Lawrence Berkeley National Laboratory, Berkeley, CA 94720
DePaolo, D J (depaolo@eps.berkeley.edu) , Department of Earth
and Planetary Science, University of California, Berkeley,
CA 94720-4767
DePaolo, D J (depaolo@eps.berkeley.edu) , Earth Science Division,
E.O. Lawrence Berkeley National Laboratory, Berkeley, CA 94720
Kennedy, B M (bmkennedy@lbl.gov) , Earth Science Division,
E.O. Lawrence Berkeley National Laboratory, Berkeley, CA 94720
Christensen, J N (jnchristensen@lbl.gov) , Earth Science Division,
E.O. Lawrence Berkeley National Laboratory, Berkeley, CA 94720
We present new isotopic
and trace element data on post-shield alkalic lavas from the
Hualalai, Mauna Kea, and Kohala. These small-volume eruptions,
which presumably correspond to small-volume source regions
in the mantle, serve as high resolution probes of geochemical
heterogeneity to complement data available from shield-stage
tholeiites that originate in the primary melting region. The
post-shield isotopic ratios average over mantle volumes as
much as 100 times smaller than those of the shield stage lavas.
The post-shield volcanic vents are spread over an area of
about 2500 km2 on the island of Hawaii. The locations extend
about 35km on either side of the axis of the Hawaiian ridge
and 60 to 110 km northwest of the centroid of the main melting
anomaly (located between Kilauea and Loihi). Helium isotopic
ratios were measured on olivine separates and, where present,
pyroxene separates from the same samples. The Sr, Nd, and
U-series isotopes were done on whole rock powders of the same
samples. He isotopes range from 6-11 R/Ra, $\^{87/86}$Sr varies
from 0.70345-0.70374, and $\epsilon$Nd from +5.3 to +7.4.
The total range of Sr and Nd isotopic variations in these
lavas is about twice that observed in the 2.84 km section
of Mauna Kea drilled by HSDP, and similar to the range encompassed
by Mauna Loa and Mauna Kea tholeiites excluding those erupted
from ML since 30 Ka. For He the range is much smaller in the
post-shield lavas than in the shield lavas. There is general
SW - NE asymmetry for all three isotope systems that could
be viewed either as the Loa-Kea dichotomy or a reflection
of the overall radial zoning of the plume. The amplitude of
Sr and Nd heterogeneities is not markedly larger than in the
shield sections of the volcanoes, which indicates that if
there are larger amplitude variations in the plume, they are
substantially smaller than the source regions of the post
shield lavas. There is no evidence that the post-shield lavas
are affected substantially by lithospheric interaction or
that they are melted from isotopically anomalous material
associated with pyroxene-rich domains. Back
V51B-0552
Generation
of Primary Kilauea Magmas: Constraints on Pressure, Temperature
and Composition of Melts
* Gudfinnsson,
G H (g.gudfinnsson@gl.ciw.edu) , Geophysical Laboratory, 5251
Broad Branch Rd, NW, Washington, DC 20015-1305 United States
Presnall, D C (presnall@gl.ciw.edu) , Geophysical Laboratory,
5251 Broad Branch Rd, NW, Washington, DC 20015-1305 United
States
Presnall, D C (presnall@gl.ciw.edu) , Dept of Geosciences,
Univ of Texas at Dallas, P.O. Box 830688, Richardson, TX 75083-0688
United States
Picrite glasses from
the submarine extension of Kilauea, Puna Ridge, which contain
up to 15.0 wt$%$ MgO, are the most magnesian glass samples
reported from Hawaii. Their compositions form a distinct olivine
fractionation trend. A comparison of this trend with phase
relations of garnet lherzolite in the CaO-MgO-Al$_{2}$O$_{3}$-SiO$_{2}$
(CMAS) and CaO-MgO-Al$_{2}$O$_{3}$-SiO$_{2}$-Na$_{2}$O-FeO
(CMASNF) system indicates that melts parental to the Hawaiian
picrites are produced by melting of a garnet lherzolite source
at a pressure of 5 $\pm$ 1 GPa. The primary melt composition
for Kilauea proposed by Clague {\it et al.} (1995), which
has 18.4 wt$%$ MgO, is close to the expected 5 GPa melt composition.
By using the pressure-independent CMASNF geothermometer (Gudfinnsson
and Presnall, 2001), we obtain a temperature of formation
of 1450$\deg$C for the most magnesian Puna Ridge glass after
correction for the presence of 0.4 wt$%$ H$_{2}$O and 0.7
wt$%$ CO$_{2}$. This assumes that the glass is not much modified
after separation from the lherzolite source. However, comparison
with phase relations in the CMAS system strongly suggests
that the most magnesian Puna Ridge glasses are the product
of some olivine fractionation, and therefore give temperature
considerably lower than that of the source. When applied to
the proposed Kilauea primary melt composition of Clague {\it
et al.} (1995), the CMASNF geothermometer gives a melting
temperature of 1596$\deg$C or about 1565$\deg$C after correction
for the presence of volatiles. This compares well with the
anhydrous solidus temperature of 1600 $\pm$ 15$\deg$C at 5
GPa for the fertile KR4003 lherzolite (Lesher {\it et al.},
2003), which has the complete garnet lherzolite phase assemblage
present at the solidus at this pressure. This consistency
supports use of phase relations from the CMAS system and the
CMASNF geothermometer to the Puna Ridge picrite compositions.
With the pressure and temperature of melting known, one can
calculate the potential temperature of the Hawaiian mantle,
provided certain conditions are met. The calculation assumes
that the temperature at the point of melt segregation is close
to the temperature of the solid adiabat. If extensive melting
has occurred prior to the segregation, this will be incorrect.
Secondly, it is assumed that the melting is occurring at the
non-conducting part of the geotherm. Provided this is the
case and the Kilauea primary melt composition truly represents
a near-primary melt composition, we derive a potential temperature
for the mantle beneath Kilauea of about 1500$\deg$C. The very
high temperature and pressure conditions for magma generation
at Hawaii appear to be unmatched by any other currently active
volcanism on the Earth. Thus, of all the candidates for plume
status, Hawaii appears to be the most robust. Back
V51B-0553
Geochemical
Constraints on the Enriched End-member of the Hawaiian Plume:
Temporal Geochemical Variation Within the Koolau Shield
* Huang, S (huangs@mit.edu)
, Department of Earth, Atmospheric and Planetary Sciences,
Massachusette Institute of Technology, 77 Mass Ave, Cambridge,
MA 02139 United States
Frey, F A (fafrey@mit.edu) , Department of Earth, Atmospheric
and Planetary Sciences, Massachusette Institute of Technology,
77 Mass Ave, Cambridge, MA 02139 United States
The surface of Koolau
volcano is composed of Makapuu-stage shield lavas which define
the well known, distinctive endmember composition for Hawaiian
shield lavas, known as the Koolau component. From $\sim$300
m to 470 m below this surface, drilling and coring by the
Koolau Scientific Drilling Project shows that the Koolau shield
lavas transitioned to a composition similar to Mauna Loa lavas;
these Koolau lavas form the Kalihi-stage of the Koolau shield.
Among all Koolau shield lavas and within the Makapuu- and
Kalihi-stages, there are strong correlations between the compositional
parameters, SiO$_{2}$ content (adjusted to be in equilibrium
with Fo$_{90}$ olivine), Sr/Nb, La/Nb and Th/La, with radiogenic
isotope ratios of Nd, Hf and Pb. These trends show that as
the shield aged there was an increasing role for a marine
sediment component accompanied by SiO$_{2}$-rich (dacite)
melt. Therefore a recycled oceanic crust component was increasingly
important as the Koolau shield moved away from the hotspot
and encountered lower temperature. Back
Click here for preprint
of Huang, S.
& F.A. Frey, Temporal geochemical variation within the
Koolau shield: A trace element perspective, submitted to Cont.
Min. Pet., Sept. 2004.
V51B-0554
New Seismic
Constraints for the Yellowstone Hotspot
Dueker, K G (dueker@uwyo.edu)
, Department of Geology and Geophysics University of Wyoming,
Dept. 3006 1000 University Ave., Laramie, WY 82071 United
States
* Schutt, D L (schutt@uwyo.edu) , Department of Geology and
Geophysics University of Wyoming, Dept. 3006 1000 University
Ave., Laramie, WY 82071 United States
Yuan, H (yuan@uwyo.edu) , Department of Geology and Geophysics
University of Wyoming, Dept. 3006 1000 University Ave., Laramie,
WY 82071 United States
Fee, D (fee@uwyo.edu) , Department of Geology and Geophysics
University of Wyoming, Dept. 3006 1000 University Ave., Laramie,
WY 82071 United States
A synthesis of recent
University of Wyoming studies of the Yellowstone Hotspot is
presented; this includes teleseismic body wave tomography,
transition zone discontinuity structure, and surface wave
tomography. Our primary conclusion is that the Yellowstone
Hotspot is not a purely "top driven" system. This conclusion
is supported by the following constraints. First, the P-wave
tomography shows a 120 km diameter low velocity pipe that
is reliably imaged to extend from beneath the current hotspot
location at Yellowstone Park down to 410 km depth. Below this
depth, resolution of a continuation of this pipe is equivocal.
It is worth noting that the pipe is tilted about $10\deg$
towards the NW. Translation of the velocity anomalies into
temperature suggests a $150-200\deg$ anomaly. Second, imaging
of discontinuity topography on the 410 discontinuity finds
a 15-20 km depression in the 410 that is spatially well correlated
with the low velocity pipe at 410 km depth. Translation of
this 410 depression into its corresponding thermal field suggests
a $150-200\deg$ anomaly. However, while the 660 km discontinuity
does show significant topography, there is no corresponding
upwarp of the 660 consistent with extension of the low velocity
pipe through the 660. In addition, stacks of the radial component
receiver functions intermittently require a 4-6% negative
velocity discontinuity at 720 km. Tangential component receiver
functions show similar magnitude arrivals from both 660 and
720 km depth. Modeling suggests that dipping layers are not
creating this tangential energy and instead an anisotropic
layer between 660-720 km is required. Third, Rayleigh wave
tomography reveals that the Yellowstone hotspot track is underlain
by extremely slow mantle between 60-120 km depth (i.e., 12%
lower than the minimum velocity found under Hawaii). This
mantle is significantly slower than a normal adiabatic profile
would predict, and significant partial melting is indicated.
The depth extent of the low velocity zone indicates the solidus
is crossed at a mean depth of 104 km. Assuming an anhydrous
solidus, this depth implies mantle temperatures $100\deg$
in excess of a normal mantle adiabat. Integration of these
new results suggests that the Yellowstone hotspot is a transient
thermal upwelling. We speculate that this upwelling is nucleated
from super-adiabatic mantle ponded below the 660 km discontinuity
in the uppermost lower mantle. Back
V51B-0555
Yellowstone
Hotspot Melting And Its Relation To Pre-Existing Crustal Structures
And Great Basin Extension
* Glen, J M (jglen@usgs.gov)
, U.S. Geological Survey, 345 Middlefield Rd., MS 989, Menlo
Park, CA 94025
Ponce, D A , U.S. Geological Survey, 345 Middlefield Rd.,
MS 989, Menlo Park, CA 94025
Sepulveda, E , Laboratoire d'Optique Physique, ESPCI, 10 Rue
Vauquelin, Paris, 75005 France
Topography and geophysical
data suggest that the path of Yellowstone hotspot (YSHS) volcanism
was controlled by pre-existing crustal structures associated
with the Snake River Plain (SRP), and that Great Basin (GB)
extension is intimately tied to hotspot melting. From its
point of inception (Glen and Ponce, 2002), the YSHS migrated
south to the southern Snake River Plain (SRP) where it began
a steady migration northeast along the eastern SRP to its
present position under the Yellowstone caldera. In doing so,
however, it had to move through a 90$\deg$ counter-clockwise
turn that is not consistent with a fixed hotspot and its predicted
path based on plate motions, and assuming a fixed hotspot.
We present evidence suggesting that the SRPs western and eastern
branches form a continuous deep crustal structure that guided
YSHS volcanism along a track from its inception in eastern
Oregon to its present position under the Yellowstone caldera.
The western and eastern segments of the SRP, which are interpreted
to have different origins and ages, nonetheless form a single
topographic depression that curves 180$\deg$ along a circular
arc. Also associated with the SRP is a broad and continuous
gravity anomaly indicating a relatively deep-seated crustal
structure extending across the western and eastern SRP. Heat
flow data, which show an uninterrupted corridor of high heat
flow values extending from Yellowstone caldera through the
SRP to the inferred inception point of the hotspot, might
reflect either the thermal footprint of the hotspot's path
or the control on heat flow by a regional-scale crustal discontinuity.
While the path of the hotspot could have been directed by
crustal structures, the location and timing of mid- to late-Tertiary
extension in the GB might, in turn, have been controlled by
hotspot melting. Well known is the age-progressive hotspot
track along the eastern SRP, presently marked by active volcanism
at the Yellowstone caldera. Less well known, is a second age-progressive
track trending northwest across the Oregon Plateau ending
at the historically active Newberry craters. The present locations
of these active melting fronts, at Yellowstone and Newberry,
coincide with the eastern and western margins, respectively,
of the GB. This remarkable correlation, while suggesting a
link between hotspot melting and GB extension, does not reveal
whether melting controls the bounds of GB extension or whether
GB extension controls the propagation of hotspot volcanism.
Another characteristic of GB extension, however, that might
reflect a causal relation between magmatism and rifting, is
the geometry and orientation of basins and ranges in the GB.
The trends of basins and ranges fan out from north-northwest
in the eastern GB to northeast in the west. When extrapolated,
these trends intersect near the SRP close to where the age-progressive
YSHS trend began $\sim$ 12-14 Ma, suggesting a relationship
between the formation of the GB and this period of the hotspot's
path. We infer that this fracturing pattern is related to
the SRP, perhaps induced by the $\sim$ 12-14 m.y. old hotspot,
and that subsequent extension in the GB exploited these pre-existing
crustal weaknesses. Back
V51B-0556
Tomographic
Images of the Yellowstone Hotspot Structure
* Jordan, M (mjordan@mines.utah.edu)
, University of Utah, 135 South 1460 East, Salt Lake City,
UT 84103 United States
Smith, R B (rbsmith@mines.utah.edu) , University of Utah,
135 South 1460 East, Salt Lake City, UT 84103 United States
Waite, G P (waite@usgs.gov) , USGS, 345 Middlefield Road,
Menlo Park, CA 94025 United States
The Yellowstone hotspot
has extensive and youthful caldera forming volcanism, very
high heatflow and significant gravity and geoid field anomalies.
It is considered as a notable example of a continental plume.
The existence of a plume beneath Yellowstone has been questioned
and debated. We present the results of a high resolution 3D
teleseismic tomography study that images the upper mantle
beneath Yellowstone down to ~800 km depth. Because the delaytime
tomography usually suffers from poor near surface resolution,
the inversion is constrained by {\it a priori information},
including a large upper crustal low-velocity structure, the
topography of the Moho, and the 410 and 660 km mantle discontinuities.
Our derived model is consistent with various data sets and
models for the Yellowstone region. In addition, long wave-length
Bouguer gravity data is used as additional constraints, which
are incorporated into the inversion via a joint inversion
approach. Our modeling has resolved a region of low upper
mantle velocity extending into, but not below, the transition
zone. The results are discussed in terms of plumes and consistency
with global models. Back
V51B-0557
Reconciling
Observations of the Yellowstone Hotspot with the Standard
Plume Model
* Ihinger, P D
(ihinger@uwec.edu) , University of Wisconsin-Eau Claire, Geology
Dept., Eau Claire, WI 54701 United States
Watkins, J M (watkinjm@uwec.edu) , University of Wisconsin-Eau
Claire, Geology Dept., Eau Claire, WI 54701 United States
Johnson, B R (johnsbre@uwec.edu) , University of Wisconsin-Eau
Claire, Geology Dept., Eau Claire, WI 54701 United States
The Yellowstone hotspot
represents the type example of plume magmatism in the continental
setting. The propagation of large silicic magmatic centers
along the Snake River Plain independently tracks the southwestward
trajectory of North American plate motion over the last 13
My. Structural deformation associated with the hotspot track
is consistent with thermal upwelling, and tomographic studies
image a well-defined cylindrical conduit at least down to
the mantle transition zone. Furthermore, the high 3He/4He
signatures suggest a deep mantle origin for Yellowstone magmas.
Yet, there are several observations of the Yellowstone region
that do not fit the standard plume model for hotspot magmatism.
These include: 1) prevalent coeval magmatism in and around
the hotspot track that continued well after passage of the
underlying plume, 2) significant bimodal magmatism that occurred
throughout the Great Basin during this time, and 3) the outpouring
of the Miocene Columbia River flood basalts (CRFB) well north
of the hotspot track. These features have led a number of
researchers to favor a shallow upper mantle origin for Yellowstone
hotspot activity controlled by structural weaknesses in the
continental lithosphere. Here, we propose that the observations
listed above conform to the standard plume model by considering
interaction of the Yellowstone plume with the descending Farallon
Plate beginning at 80 Ma. Anomalous geologic activity occurred
throughout the Cenozoic Era in the North American Cordillera
(NAC) and must be addressed in any model for the origin of
magmatism in the western US, including the Yellowstone hotspot.
In particular, extensive field and geochemical studies document
a pronounced eastward migration of deformation and magmatism
throughout the NAC from 80 to 40 Ma. Most researchers attribute
this activity to shallowing of the Farallon slab beneath NA
at this time. In addition, geochemical studies in the NAC
document a transition in magmatism from predominantly calc-alkaline
(associated with ancient slab-derived fluids within the sub-continental
lithosphere) to predominantly tholeiitic (with distinctive
OIB signatures). This transition has been attributed to the
eventual foundering of the shallow slab with replacement by
`asthenosphere'. Here, we document that magmas with OIB affinity
are observed throughout the Cenozoic in the NAC, often before
a documented `transition'. We show that these magmas are primarily
binary mixtures of two well-known mantle plume components
EMI and FOZO. In our model, we propose that the Yellowstone
starting plumehead impinged beneath the subducting Farallon
Plate at 80 Ma and spread laterally while continuing to ascend.
Magmas with OIB affinity erupted only after penetration of
the plume through the cold, rigid Farallon slab. In this way,
the CRFB, at only 10% of the eruptive volume of typical flood
basalt provinces, represent partial melting of only a fraction
of the original Yellowstone starting plumehead. Evidence of
additional leakage of the plume is found in the Chilcotin
flood basalts in BC, the Crescent Terrane volcanics in the
Pacific Northwest, and kimberlites, diatremes, and widespread
basaltic flows found throughout the NAC. Collectively, the
magmatic features that seem to oppose the plume hypothesis
can be reconciled by considering a broader context for the
origin of the Yellowstone hotspot. Indeed, the `anomalous'
geologic activity observed within the NAC is anticipated by
the standard plume model; the frequency of hotspots observed
on Earth demands that some starting plumeheads will encounter
destructive plate margins and generate significant uplift,
deformation, and magmatism within a broad region of the overriding
lithosphere(s). Back
V51B-0558
New Constraints
on the Evolution of the Deccan Volcanic Province, India
* Mohan, G (gmohan@iitb.ac.in)
, Department of Earth Sciences, Indian Institute of Technology
Bombay, Powai, Mumbai, 400076 India
Ravi Kumar, M , National Geophysical Research Institute, Uppal
Road, Hyderabad, 500007 India
The evolution of the
Deccan volcanic province (DVP) of India is commonly linked
to the upwelling of a mantle plume beneath the Indian subcontinent
in the late Cretaceous. However, the proposed mechanisms ranging
from plume to non-plume models remain debatable. The pre-requisites
for the plume models are a thin lithosphere and an anomalously
hot upper mantle, which still remain uncertain in the case
of DVP. In the present study, the mantle discontinuities beneath
DVP are imaged through P-wave receiver function analysis,
using about 900 seismograms from six broadband stations deployed
along a 350km long profile in western DVP. We find that the
move out corrected P-to-s conversion times from the standard
410 and 660 km discontinuities are normal, indicating absence
of anomalous thermal anomalies in the upper mantle beneath
DVP. An additional discontinuity at 200km, corresponding to
the Lehmann discontinuity, possibly representing the base
of the lithosphere, is delineated. These results imply that
the lithosphere beneath DVP is normal and not thinned by an
upwelling mantle plume. This, coupled with pervasive presence
of a sub-Moho low velocity zone, raises doubts about the plume
impact/incubation models suggested for volcanism in DVP, favoring
rifting as a more plausible mechanism. Back
V51B-0559
Hafnium-Osmium
Systematics of Cretaceous Group II Kimberlites from India
Kent, R W (r.w.kent@lboro.ac.uk)
, Loughborough University, Rutland Hall, Loughborough, LE11
3TU United Kingdom
* Ingle, S (single@geo.titech.ac.jp) , EPS, Tokyo Inst. Tech.,
2-12-1 Ookayama, Meguro-ku, Tokyo, 152-8551 Japan
Mattielli, N (nmattiel@ulb.ac.be) , DSTE, Univ. Libre de Bruxelles,
50, Av. FD Roosevelt, Brussels, B-1050 Belgium
Kempton, P D (pdk@nerc.ac.uk) , NERC, North Star Avenue, Swindon,
GBR SN2 1EU
Saunders, A D (ads@leicester.ac.uk) , Dept. Geology, University
of Leicester, University Road, Leicester, LE1 7RH United Kingdom
Suzuki, K (katz@pop.jamstec.go.jp) , IFREE, JAMSTEC, 2-15
Natsushima-Cho, Yokosuka-city, Kanagawa, 237-0061 Japan
Hafnium and Os isotopes
of orangeites (Group II kimberlites) may prove to be a useful
tool in deciphering mantle sources and petrogenetic histories.
According to recent interpretations, orangeite parent magmas
are derived from a source with high Os concentrations, ocean
island basalt-like gamma Os values (i.e. chondritic to slightly
suprachondritic), and low time-integrated Lu/Hf relative to
Sm/Nd (Pearson et al., 8th IKC, 2003; Nowell et al., J. Petrol.,
2004). Thus, in Hf-Nd isotope space, data for orangeites generally
plot below the terrestrial array, as defined by oceanic basalts
and continental crust. Osmium isotope compositions may reflect
a source for orangeite parent magmas in the convecting mantle,
and subsequent interactions between these magmas and an Os-poor,
radiogenic source. In order to evaluate this petrogenetic
model, we studied a suite of mid-Cretaceous orangeites from
eastern India, rocks that have been linked in space and time
to the Kerguelen hot spot. High-precision Lu-Hf and Re-Os
isotope data were obtained by MC-ICPMS and N-TIMS, respectively.
The Indian orangeites have Hf isotopic compositions ranging
from chondritic to moderately subchondritic. In Hf-Nd isotope
space, data for these samples plot within the terrestrial
array, on the very low end of the ocean basalt range. Osmium
concentrations are high and Os isotopic values fall mostly
within the range of present-day ocean island basalts. In detail,
our preliminary Os isotopic data appear to reflect mixing
between an Os-rich, chondritic mantle source and an Os-poor,
suprachrondritic contaminant. The Os isotopes are not obviously
correlated with Hf, Nd, Sr or Pb isotopes in the Indian orangeites;
this might imply that their parent magmas interacted with
continental crust on the way to the surface. Alternatively,
the radiogenic, Os-poor component could also be the cause
of the lithophile isotope ratios. Back
V51B-0560
K-T magmatism
of western Rajasthan, India: Manifestation of Reunion plume
activity or extensional lithospheric tectonics?
* Sharma, K (sharmasirohi@yahoo.com)
, Kamal Kant Sharma, Department of Geology Government PG College,
SIROHI, RAJ 307001 India
A number of alkaline
plutons have been recorded at the K-T (Cretaceous-Tertiary)
boundary in western Rajasthan, India. Significant magmatism
occurred at Mundwara, Barmer, Sarnu-Dandali and Tavider. The
evolution of the Cambay-Sanchor-Barmer rift during the K-T
period resulted in these alkaline complexes at the rift margins.
Sedimentary basins are developed in the Barmer and Jaiselmer
regions. The magmatism of Mundwara and Sarnu-Dandali is dated
at 68.50 Ma and considered as an early pulse of Deccan volcanism.
Several workers correlated K-T sedimentary basin evolution,
magmatism and other tectonic features of western Rajasthan
with the Reunion plume-interaction in the northwestern Indian
shield. Alkaline igneous complexes along the rift from the
southern part are reported from Phenai Mata, Amba Dongar and
Seychelles. The Seychelles was part of the northwestern Indian
shield prior to Deccan volcanism. The Mundwara igneous complex
represents three distinct circular plutonic bodies - Toa,
Mer and Mushala, which are situated in the periphery of an
area three kilometers in radius. Besides these, there are
numerous concentric and radial dykes of lamprophyre, carbonatite,
dolerite and amphibolite. All these three bodies represent
different phases of intrusion and are not similar to each
other. The alkaline rocks of Sarnu-Dandali occur as dykes
and isolated plugs in the desert sand. Carbonatite dykes are
also reported from southeast of Barmer. The Tavider outcrop
is devoid of any plutonic rock and consists of rhyolite, andesite
and basalt. These rocks occur along the Precambrian Malani
magmatic lineaments. The development of the Cambay-Sanchor-Barmer
rift caused reactivation of Precambrian fractures and resulted
in magmatism at the basin margin. The Gondwanaland fragmentation
during the Mesozoic era caused extensional tectonics in the
northwestern Indian shield. This led to the development of
rift basins in Gujarat and western Rajasthan. Deccan volcanism,
separation of the Seychelles microcontinent from India, sedimentary
basin development in western Rajasthan and the alkaline magmatism
of Mundwara, Sarnu-Dandali and elsewhere are considered to
be the products of Reunion plume activity in western India.
However, basin development began in western Rajasthan in the
Jurassic period and no plume has been suggested for this.
The continual extensional tectonic regime caused deep fractures
in the continental and oceanic lithosphere. The Cambay-Sanchor-Barmer
rift developed in continental lithosphere. The Mundwara, Sarnu-Dandali
and Barmer magmatism with nephelinite-carbonatite affinity
at the basin margin represents a typical rift-tectonic setting.
The tectonic setting and crustal development during the K-T
period in western Rajasthan represents an extensional tectonic
regime rather than the manifestation of Reunion plume activity.
Back
V51B-0561
Implications
for the Emplacement of the Deccan Traps (India) From Isotopic
and Elemental Signatures of Dikes
* Vanderkluysen,
L (loyc@hawaii.edu) , VGP, SOEST University of Hawaii at Manoa,
POST #606, 1680 East-West Road, Honolulu, HI 96822 United
States
Mahoney, J J (jmahoney@hawaii.edu) , VGP, SOEST University
of Hawaii at Manoa, POST #606, 1680 East-West Road, Honolulu,
HI 96822 United States
Hooper, P R (prhooper@mail.wsu.edu) , Department of Geology
Washington State University, P.O. Box 642812, Webster 1228,
Pullman, WA 99164-2812 United States
A large swarm of dikes
with no preferred orientation in the western Deccan Traps
of India, termed the Nasik-Pune swarm, has been interpreted
by previous workers (Beane et al., 1986, Bull. Volcanol. 48,
p. 61; Hooper, 1990, Nature 345, p. 246) to be the principal
locus of feeders for the massive lava pile, and the lack of
preferred orientation has been interpreted as strong evidence
that the main phase of eruptive activity was not accompanied
by significant directed extension of the regional lithosphere.
Large dike swarms along the western coast and in the Narmada-Tapti
graben in the northern Deccan show strong preferred orientations
but have generally been discounted as major feeder systems.
An isotopic, major element, and trace element study of dikes
of the Nasik-Pune and coastal swarms demonstrates that the
situation is more complex. About half of our samples display
isotopic compositions that are not observed in the western
Deccan lava pile, despite having major and trace element similarities
to some of the lavas. The other half show isotopic and elemental
compositions matching those of the upper formations of the
lava pile (i.e. the Poladpur, Ambenali and Mahabaleshwar formations).
Both categories of dikes are present in both the Nasik-Pune
and coastal swarms. Thus far, only a single dike can be correlated
unambiguously with the stratigraphically lowest formation,
the Igatpuri-Jawhar; this dike is in the Nasik-Pune swarm.
The combined results suggest that the coastal and Nasik-Pune
swarms were both feeder zones for the Deccan Traps. The dikes
having strong affinities with the Ambenali and Poladpur formations
(two of the most extensive formations in the lava pile) appear
to be preferentially, although only weakly, N-S oriented.
The implication is that east-west extension may have begun
by the time of the upper-formation eruptions. Back
V51B-0562
New Age and
Geochemical Data From Seamounts in the Canary and Madeira
Volcanic Provinces: A Contribution to the "Great Plume Debate"
* Geldmacher,
J (jgeldmacher@ifm-geomar.de) , IFM-GEOMAR, Geb. Ostufer,
Wischhofstr. 1-3, Kiel, 24148 Germany
Hoernle, K (khoernle@ifm-geomar.de) , IFM-GEOMAR, Geb. Ostufer,
Wischhofstr. 1-3, Kiel, 24148 Germany
van den Bogaard, P (pbogaard@ifm-geomar.de) , IFM-GEOMAR,
Geb. Ostufer, Wischhofstr. 1-3, Kiel, 24148 Germany
Duggen, S (sd@dlc.ku.dk) , Danish Lithosphere Centre, Oster
Voldgade 10, Copenhagen, 1350 Denmark
Werner, R (rwerner@tethys-geoconsulting.de) , TETHYS Geoconsulting
Gmbh, Wischhofstr. 1-3, Kiel, 24148 Germany
The role of hotspots
(mantle plumes) in the formation of intraplate volcanic island
and seamount groups is being increasingly questioned, in particular
concerning the abundant and somewhat irregularly distributed
island and seamount volcanoes off the coast of northwest Africa.
However, new $^{40}$Ar/$^{39}$Ar ages and Sr-Nd-Pb isotope
geochemistry of volcanic rocks from two seamounts northeast
of the Canary Islands and two northeast of the Madeira Islands
provide new support for the plume hypothesis. The oldest ages
of shield stage volcanism from seamounts and islands northeast
of the Canary and Madeira Islands confirm progressions of
increasing age to the northeast for both island/seamount chains
consistent with northeast directed plate motion. Calculated
angular velocities for the average movement of the African
plate in both regions gave similar values of about 0.45\deg
plus/minus 0.05\deg/Ma around a rotation pole located north
of the Azores Islands. Furthermore, the curvature of the chains
clearly deviates from the E-W orientation of fracture zones
in the East Atlantic. A local control of surface volcanism
by lithospheric zones of weakness, however, is likely for
some E-W elongated seamounts and islands. The isotope geochemistry
additionally confirms that the two volcanic provinces are
derived from distinct sources, consistent with distinct mantle
plumes having formed both volcanic groups. Conventional hotspot
models, however, cannot easily explain the wide distribution
of seamounts in the Canary region and the long history of
volcanic activity at single volcanic centers (e.g. Dacia seamount,
47-4 Ma; Selvagen Islands, 30-3 Ma). A possible explanation
could involve interaction of a Canary mantle plume with small-scale
upper mantle processes such as edge driven convection at the
edge of the NW African craton (e.g. King and Ritsema, 2000,
Science 290, 1137-1140). Back
V51B-0563
Ascension Island,
South Atlantic: Deep Plume or Shallow Melting Anomaly?
* Weaver, B (bweaver@ou.edu)
, Barry Weaver, Geology and Geophysics University of Oklahoma,
Norman, OK 73071 United States
Ascension Island ($7\deg$
56' S, $14\deg$ 22' W) is the sub-aerial manifestation of
a broad region of anomalous volcanism on, and adjacent to,
the South Atlantic MAR between the Ascension Fracture Zone
(AFZ; approx. $7\deg$ S) and the Bode Verde Fracture Zone
(BVFC; approx. $11.5\deg$ S). Zero age MORB from the MAR in
this region have Pb isotope compositions more radiogenic than
N-MORB and which plot significantly above (positive $\Delta$8/4)
the Northern Hemisphere Reference Line (NHRL), and have La$_{N}$/Sm$_{N}$
higher and Zr/Nb lower than N-MORB. The enriched character
of the MORB volcanism has been suggested to be the product
of a plume located near Circe Seamount, at approx. $9\deg$
S, $11.7\deg$ W, or of a plume located beneath two large on-axis
seamounts at $9\deg$ 50' S on the MAR. The volcanic rocks
of Ascension Island comprise a diverse basalt - hawaiite -
mugearite - benmoreite - trachyte suite. There is significant
chemical heterogeneity in the mafic lavas, with high Zr/Nb,
intermediate Zr/Nb, low Zr/Nb, and Dark Slope Crater lava
types reflecting significant source heterogeneity, as indicated
by trace element and Sr-Nd-Pb isotope systematics. Data for
samples from shallow boreholes and one deep borehole suggest
that high Zr/Nb lavas dominate in the subsurface and that
there has been a temporal trend toward eruption of increasingly
enriched, and more diverse, lava compositions with growth
of the volcanic edifice. Ascension Island volcanic rocks have
Pb isotope compositions which plot significantly below (negative
$\Delta$8/4) the NHRL and trend toward St. Helena HIMU compositions.
In addition to Ascension Island, there are numerous seamounts
(Circe, Grattan, Stvor, etc.) both on and off the MAR axis
between the AFZ and the BVFZ. The locations of the seamounts
are closely associated with fracture zones and do not reflect
the directions of absolute motion of the South American and
African plates (for example, the two large seamounts to the
west of Ascension Island are on a flow line parallel to the
AFZ). The nature of the distribution of Ascension Island and
seamounts between the AFZ and BVFZ does not conform to the
classic deep plume model. The geochemistry of Ascension Island
and MORB lavas is best explained as the result of melting
of shallow chemically heterogeneous mantle, with bursts of
excess magmatism when "pods" of enriched (HIMU-type component)
mantle pass into the sub-axial MAR melting zone, and with
focussing of magma supply being controlled by fracture zone
distribution. Back
V51B-0564
Tristan-Gough
Plume: Negative Ce-Anomalies as Evidence of a Recycled Sediment
Component in the Deep Mantle
* Class, C (class@ldeo.columbia.edu)
, Lamont-Doherty Earth Observatory, POB 1000 RT 9W, Palisades,
NY 10964 United States
le Roex, A (aleroex@geology.uct.ac.za) , University of Cape
Town, Private Bag, Rondebosch, 7701 South Africa
Recent volcanism on
Tristan and Gough Islands, the associated age-progressive
volcanism forming the Rio Grande Rise and Walvis Ridge symmetric
to the Mid-Atlantic Ridge and extending towards the large
Paran\'{a} (S America) and Etendeka (Africa) flood basalt
province, as well as the near uniform composition of the related
mantle source since $\sim$130 Ma cannot be reconciled with
models other than the plume model. We thus consider Tristan-Gough
basalts as probes for deep mantle sources. New high precision
trace element data (acquired by ICPMS) on mafic volcanic rocks
from Gough Island reveal variable depletion in Ce relative
to adjacent rare earth elements when normalized to chondrites,
i.e. so-called negative Ce anomalies (Ce/Ce*$<$1). Ce/Ce*
values in Gough lavas extend down to values of $\sim$0.93.
The magnitude of the anomaly varies with other chemical parameters
(e.g. with $^{87}$Sr/$^{86}$Sr ratios) and is therefore not
an analytical artifact. Correlations of Ce anomalies with
$^{87}$Sr/$^{86}$Sr ratios has been taken as evidence for
alteration by sea-spray, but in Gough basalts the accompanying
alteration-induced rare earth element enrichment is missing.
Thus Ce anomalies in Gough samples are not a weathering phenomenon.
Ce anomalies are only formed in the oceans, being characteristic
of certain deep sea sediments. Positive Pb and negative Nb
anomalies, typical for such sediments, are absent in Gough
Island lavas and thus preclude a contribution from sediment
within the volcanic pile. Whereas the negative Ce anomaly
combined with anomalous isotope ratios and higher LIL/REE
strongly indicates a sediment component, the absence of a
significant Nb depletion or Pb enrichment shows that the sediment
component has been modified during subduction, before it became
part of the source of some Gough Island lavas. The presence
of a negative Ce anomaly is the strongest indication yet for
recycled sediment in the Tristan-Gough plume source. The data
confirm that the processes occurring on the Earth's surface
affect the evolution of the deep earth. Back
V51B-0565
The Origin
of EM1 Signatures in Basalts From Tristan da Cunha and Gough
* Stracke, A (stracke@mpch-mainz.mpg.de)
, Max-Planck-Institut fuer Chemie, Postfach 3060, Mainz, 55020
Germany
Willbold, M , Max-Planck-Institut fuer Chemie, Postfach 3060,
Mainz, 55020 Germany
Hemond, C , UBO-CNRS UMR 6538 Domaines oceaniques IUEM, Place
Nicolas Copernic, Plouzane, 29280 France
A long-standing hypothesis
is that enriched mantle 1 (EM-1)-type ocean island basalt
(OIB) sources contain pelagic sediments. Pelagic sediments
range in composition from clays to calcareous or siliceous
oozes and encompass a wide range of chemical compositions
[1]. For geochemical purposes the use of the term pelagic
sediments is often restricted to a special group of pelagic
sediments with distinctive enrichment of Rare Earth Elements
(REE). The geochemical composition of such REE-enriched pelagic
sediments, however, is by no means representative of the geochemical
composition of pelagic sediments in general. The extremely
high REE/non-REE element ratios in REE-enriched pelagic sediments
(e.g. high Lu/Hf, Sm/Hf, La/Nb, La/Th, Eu/Ti, and Gd/Ti ratios)
translate into high $^{176}$Hf/$^{177}$Hf ratios for given
$^{143}$Nd/$^{144}$Nd ratios with time. OIB sources containing
this special variety of REE-enriched pelagic sediment should
therefore plot above the oceanic basalt array and mixing arrays
with these sources are expected to have a shallow slope in
a Hf-Nd isotope diagram. Here we present new Hf-Nd isotope
and trace element data for EM-1-type OIB from Tristan da Cunha
and Gough in the South Atlantic Ocean. The samples from Tristan
have a small range in Hf-Nd isotopic composition and plot
within the oceanic basalt array in a Hf-Nd isotope diagram.
Samples from Gough form a trend with a slope slightly steeper
than that of the ocean basalt array in a Hf-Nd isotope diagram.
OIB in general have a very restricted range in Gd/Ti and Sm/Hf
ratios, and high La/Nb are associated with low Lu/Hf ratios.
In detail, samples from Tristan and Gough have the lowest
Lu/Hf and highest La/Nb ratios. Thus from the combined Hf-Nd
isotope and trace element composition of basalts from Tristan
and Gough involvement of this special variety of (REE-enriched)
pelagic sediments can be excluded. Similar observations are
made, and thus similar arguments hold, for other EM-1-type
localities (Walvis ridge [2] and Pitcairn island [3]). Due
to the considerable spread in geochemical composition of pelagic
or any other group of sediments (e.g. marine sediments with
a higher proportion of terrigenous components), it is difficult
to attribute characteristic elemental or isotopic signatures
to certain groups of sediment. Moreover, subducting sediments
are complex mixtures of different types of sediment [1]. Thus
it is difficult to find unique evidence either in favor of
or against the involvement of sediments in general at Tristan
and Gough, or any other individual OIB locality. Also, it
appears highly unlikely that sub-arc processing has an equalizing
effect on the composition of different subducting sediments
[4]. Associating the similar isotopic characteristics of certain
OIB groups and/or mantle-end-members (e.g. EM-1) to recycled
sediments is therefore also problematic. [1] Plank, T. and
C. H. Langmuir, Chem. Geol., 145, 325-394, 1998. [2] Salters,
V. J. M. and X. Li, Geochim. Cosmochim. Acta, 68, A554, 2004.
[3] Eisele, J., M. Sharma, J. G. Galer, J. Blichert-Toft,
C. W. Devey and A. W. Hofmann, Earth Plan. Sci. Lett., 196,
197-212, 2002. [4] Johnson, M. C. and T. Plank, Geochem.,
Geophys., Geosys., 1, pp. 29, 1999. Back
V51B-0566
Isotope and
Trace Element Characteristics of Walvis Ridge Basalts Argue
Against Pelagic Sediment Involvement
* Salters, V J
(salters@magnet.fsu.edu) , National High Magnetioc Field Laboratory,
1800 E Paul Dirac Drive, Tallahassee, FL 32312 United States
Li, X (xli@magnet.fsu.edu) , National High Magnetioc Field
Laboratory, 1800 E Paul Dirac Drive, Tallahassee, FL 32312
United States
Walvis Ridge is the
"type" locality for Enriched Mantle I compositions (Zindler
and Hart, 1984). We have determined the trace element and
isotopic compositions of Walvis Ridge basalts to better determine
the geochemical characteristics of the enriched mantle endmember
which is thought to include a significant contribution from
recycled pelagic sediments. Characteristic for pelagic sediment
contribution is a shallow slope on a Hf-Nd isotope correlation
diagram. For Walvis Ridge basalts epsilon Hf varies from -2
to +12 while epsilon Nd varies from -4 to 6. This range is
larger than previously published results and results in a
well-correlated array (R2=0.97) with a steeper slope than
the general ocean island basalt (OIB) array. Walvis Ridge
Pb-isotope variation is also large $^(206)$Pb/$^(204)$Pb and
$^(207)$Pb/$^(204)$Pb range from 17.5 to 18.5 and 15.46 to
15.53 respectively. The low $^(206)$Pb/$^(204)$Pb basalts
also show low Hf and Nd isotopic characteristics and in general
Nd-Sr-Hf-Pb isotopic compositions are well correlated indicating
two component mixing. The Walvis Ridge basalts also show a
large range in trace element compositions: Ce/Yb 4.68-25.52;
Sm/Hf 1.11-1.58; La/Nb 0.72-1.63, La/Sm 1.59-5.13; U/Th 0.10-0.53
all well outside the range of what is expected for variations
related to degree of melting and crystallization. Isotopic
and trace element characteristics of the basalts are well
correlated with low epsilon-Hf basalts showing low Sm/Hf,
high Ce/Yb and high U/Th. The trace element data confirm the
isotopic data as most of their variations can also be explained
by mixing of just two components. New data obtained on basalts
from Tristan da Cunha and Gough show similar type variations
in trace element and isotopic compositions as the Walvis Ridge
basalts. Walvis Ridge is the "type" locality for Enriched
Mantle I compositions (Zindler and Hart, 1984). The isotopic
variations as well as the coupled trace element-isotope variations,
especially the steep slope on the Hf-Nd isotope correlation
diagram and the positive correlation between Sm/Hf and Hf-isotopic
composition, argue strongly against pelagic sediment involvement.
Detailed analysis of the trace element and isotope variations
show that the data is hard to reconcile with recycled components
and that intra mantle differentiation is a more likely process
explaining the variations. Back
V51B-0567
Contrasting
Styles Between the Structure and the Magmatism of the West
and South Hatton/Rockall Margins (North Atlantic Igneous Province)
* Gernigon, L
(lg@cp.dias.ie) , Marine & Petroleum Geology Research
Group Department of Geology, University College Dublin, Department
of Geology, University College Dublin Belfield, Dublin 4,
Ireland., Dublin, Dublin 4 Ireland
Ravaut, C (cr@cp.dias.ie) , Dublin Institute for Advanced
Studies (DIAS), 5, Merrion Square, Dublin, Dublin 2 Ireland
Shannon, P M (P.Shannon@ucd.ie) , Marine & Petroleum Geology
Research Group Department of Geology, University College Dublin,
Department of Geology, University College Dublin Belfield,
Dublin 4, Ireland., Dublin, Dublin 4 Ireland
Chabert, A (ac@cp.dias.ie) , Marine & Petroleum Geology
Research Group Department of Geology, University College Dublin,
Department of Geology, University College Dublin Belfield,
Dublin 4, Ireland., Dublin, Dublin 4 Ireland
O'Reilly, B M (br@cp.dias.ie) , Dublin Institute for Advanced
Studies (DIAS), 5, Merrion Square, Dublin, Dublin 2 Ireland
Readman, P W (pr@cp.dias.ie) , Dublin Institute for Advanced
Studies (DIAS), 5, Merrion Square, Dublin, Dublin 2 Ireland
The North Atlantic
rifted margins area is usually characterised by a combination
of regional uplift, crustal extension and magmatism leading
to the formation of seaward dipping reflectors (SDRs). The
temporal and spatial relationships of these volcano-tectonic
processes are interpreted as the response to a deep mantle
anomaly. Integration of new seismic, potential field and well
calibration data allows us to clarify the structure and stratigraphy
of both the West Hatton margin (WHM) and the south Rockall/Hatton
margin (SHRM). The WHM represents a Late Paleocene-Early Eocene
volcanic margin as suggested by the typical volcano-stratigraphic
sequence (Inner SDRs, Outer High, Outer SDRs, oceanic crust)
observed near the beakup axis. The Inner SDR wedges are typically
underlain by thin and intruded sediments and continental crust.
Oceanward, the deep structures of the Outer High and outer
SDRs features are underlain by a massive, uniform thick high-velocity
lower crustal body interpreted as breakup underplating that
probably controlled the SDR emplacement by a mid-crustal updoming.
To the south, the SHRM is a complex rifted zone linking the
WHM with the Celtic and Iberian non-volcanic margins. The
SHM was also influenced by an earlier Cretaceous phase of
break-up and, compared to the WHM, the nature of the continent-ocean
transition is different. Close to the C34 magnetic anomaly,
the continent-ocean transition is defined by a sharp contrast
between a smooth oceanic basement and block-faulted structure
that exhibit both syn- and post-breakup features. Close to
the oceanic crust, the complex may represent a combination
of serpentinized peridotites and/or intruded continental basement.
No SDRS are imaged, but evidence of magmatism is identified.
Thin lava flows and transgressive sill complexes, magmatic
plugs and seamounts intruding the Cretaceous oceanic crust
are observed near the SRHM. Close to the Charlie-Gibbs Fracture
Zone, the West Thulean Ridge defining the southern end of
the volcanic province is also imaged. It probably represents
a thick Paleocene oceanic volcanic plateau, uplifted and block
faulted during Cenozoic time. The age of the magmatism along
the SRHM is controversial. Evidence of Palaeogene magmatism
can be observed but Early Cretaceous igneous activity is also
indicated. The best example is the Barra Volcanic Ridge, which
is reinterpreted here as a structural horst cut by Early Cretaceous
extrusives and intrusives and subsequently by Tertiary dykes.
Detailed stratigraphic analysis documents the contrasting
subsidence/uplift patterns that differentiate the sedimentary
architecture of the WHM and the SRHM. This project is funded
by the Geological Survey of Ireland and the Irish Petroleum
Infrastructure Programme. Back
V51B-0568
Plumes are
not what they seem: Physics of big, fat, firm plumes
* Korenaga, J
(jun.korenaga@yale.edu) , Dept. of Geology and Geophysics,
Yale University, PO Box 208109, New Haven, CT 06520 United
States
In recent years, merely
philosophical and often superficial arguments regarding the
origin of hotspots and the existence of mantle plumes have
become very popular. What is lacking in this trend is constructive
discussion based on fluid mechanics constrained by geophysical
observations; plumes are fluid-mechanical phenomena, and seismic
imaging alone does not tell the whole story. By the same taken,
convection models without proper scaling to the Earth's mantle
are merely of theoretical interest. Although it is sometimes
claimed that plumes with large radii (as observed in recent
seismic tomography) have been reproduced in `realistic' mantle
convection models, this is simply incorrect. What is referred
to as realistic models actually uses either temperature-independent
viscosity or very high mantle viscosity or the combination
of both. Furthermore, those numerical plumes carry volume
flux considerably greater than inferred from surface observables.
In this presentation, I will propose a new self-contained
framework for plume dynamics that can connect seismic imaging,
fluid dynamics, and surface observations. This new framework
can also explain various puzzling aspects of the D'' layer
and hotspot magmatism. Back
V51B-0569
The mantle
potential temperature anomaly beneath Iceland is insufficient
for a thermal plume.
* Foulger, G R
(g.r.foulger@durham.ac.uk) , University of Durham, Science
Laboratories, South Rd., Durham, DH1 3LE United Kingdom
Vinnik, L P (vinnik@ifz.ru) , Institute of Physics of the
Earth, Moscow, Moscow, LEV VIN Russian Federation
Du, Z (zd202@cam.ac.uk) , Institute for Theoretical Geophysics,
Downing St., Cambridge, DB2 3EQ United Kingdom
One of the few primary
characteristics of mantle plumes is high temperature compared
with surrounding mantle. A temperature anomaly of at least
200-300 K is thought to be required for an upper-mantle plume
rising from the base of the mantle transition zone. More than
15 methods, many of them independent, have been applied to
estimate the temperature anomaly beneath Iceland. Seismic
methods include using the Vp/Vs ratio and attenuation to determine
the temperature of the crust, and using P- and S-wave mantle
tomography. P- and S-wave receiver functions have been used
to estimate the depths to discontinuities, including those
associated with the low-velocity zone and bounding the mantle
transition zone. Seismic wave travel times have been used
to estimate the velocities between the discontinuities. Petrological
methods include olivine glass geothermometry, the study of
melt inclusions in basalts, CMASNF geothermometry of high-MgO
glasses, major element systematics of Icelandic MORB and the
search for an olivine control line in Icelandic picrite cumulates.
Other methods include modeling the bathymetry of the north
Atlantic assuming it has a thermal origin, modeling the subsidence
of the ocean crust and uplift of the Hebrides shelf, and studying
ocean floor heat flow measurements. Virtually all results
either require or are compatible with a temperature anomaly
of no more than ~ 50-100 K beneath Iceland. The crust there
is cooler than that beneath the East Pacific Rise. Seismic
tomography is compatible with temperature anomalies of up
to 200 K, decreasing to about 100 K at depths greater than
200 km, where the seismic anomaly is weaker. This assumes
that compositional effects are zero and partial melt is absent,
however. Other seismic results require the presence of partial
melt, and this requires substantial downward-adjustment of
the temperature anomaly estimate from tomography. P-wave receiver
functions show that the 410-km discontinuity is warped downward
but that the 650-km discontinuity is flat. These results are
consistent with a temperature anomaly of about 100 K at 410
km and zero at 650 km. All petrological methods suggest relatively
small temperature anomalies unless olivine control is assumed
for Icelandic picrite cumulates. The validity of this assumption
is questionable. Modeling of bathymetry and vertical motions
suggests temperature anomalies of up to about 100 K or less.
Virtually all temperature estimates for the Iceland region
are thus consistent in suggesting that the temperature anomaly
beneath Iceland is modest, and insufficient for a thermal
mantle plume that rises through its own thermal buoyancy.
Back
http://www.mantleplumes.org
V51B-0570
Widespread
Synchronous Volcanism Reveals a Broad Galapagos Hotspot Melting
Anomaly
* O'Connor, J
M (John.O.Connor@falw.vu.nl) , Department of Isotope Geochemistry,
Vrije University, Amsterdam, De Boelelaan 1085, Amsterdam,
1081 HV Netherlands
Stoffers, P (pst@gpi.uni-kiel.de) , Institute for Geosciences,
Christian-Albrechts-University, Ludewig-Meyn-Str. 10, Kiel,
D-24118 Germany
Wijbrans, J R (Jan.Wijbrans@falw.vu.nl) , Department of Isotope
Geochemistry, Vrije University, Amsterdam, De Boelelaan 1085,
Amsterdam, 1081 HV Netherlands
Worthington, T J (tw@gpi.uni-kiel.de) , Institute for Geosciences,
Christian-Albrechts-University, Ludewig-Meyn-Str. 10, Kiel,
D-24118 Germany
The massive aseismic
ridges and associated seamounts dominating the morphology
of the Panama Basin, eastern Central Pacific, have long been
attributed to a Galapagos hotspot melting anomaly linked to
a deep-seated mantle plume. Although these structures can
provide information about the origin of hotspots and existence,
or otherwise, of mantle plumes very little is known about
their volcanic histories due to a lack of direct age and geochemical
information. We report here 74 whole rock and 2 plagioclase
$^{40}$Ar/$^{39}$Ar ages for rocks dredged from 53 locations
during the first systematic sampling of the Cocos, Carnegie,
Coiba and Malpelo aseismic ridges and associated seamounts
(F.S. SONNE PAGANINI expedition). In addition we also report
ages for DSDP drill sites on Cocos, Carnegie and Coiba ridges
and 7 Cocos Island subaerial samples. The distribution of
new, and published ages for the Galapagos Archipelago-platform
and NE end of the Cocos Ridge, show a general trend of increasing
age with distance from the Galapagos Archipelago. A more dominant
trend however is one of aseismic ridge-seamount formation
in a progression of broad zones of synchronous, often overlapping
volcanism created at discrete intervals. Broad zones of coeval
Cocos and Carnegie volcanism once formed much larger regions
of synchronous volcanism that have been split apart by the
complex history of seafloor spreading associated with the
Cocos-Nazca spreading center. We link these broad regions
of synchronous volcanism to a correspondingly large hotspot
melting anomaly. The present day, as yet unfragmented, zone
of synchronous volcanism associated with this proposed broad
hotspot is marked by the extensive region of recent volcanism
extending across the Nazca and Cocos plates encompassing the
Galapagos Archipelago-Platform and the Cocos Ridge as far
north as Cocos Island. The complex tectonic history of the
Cocos-Nazca spreading-center has controlled how the broad
zones of synchronous, often overlapping volcanism created
by the broad Galapagos melting anomaly have been fragmented
between the Cocos and Nazca plates. However, interplay between
the broad Galapagos melting anomaly and the Cocos-Nazca spreading
center is a second-order process compared to a fundamental
underlying mantle process responsible for a broad Galapagos
hotspot melting anomaly exhibiting long-lived characteristics
(size, time-progression, episodicity) which, on a first-order,
are independent of local tectonics and lithospheric architecture.
Evidence for a broad Galapagos hotspot melting anomaly and
the possibility of detecting long-lived underlying mantle
processes has implications for how oceanic hotspot volcanism
is sampled for purposes of rigorously testing the mantle plume
paradigm. A major question posed by our results is whether
individual Pacific seamount chains are in fact the product
of tectonic plate drift over narrow hotspots? If not, then
inferring the existence and behavior of a mantle plume on
the basis of age progression of volcanism produced by a narrow
seamount chain could well prove to be misleading. Thus, although
great leaps are being made in the theory and numerical modeling
- often on a global scale - of hypothesized deep plumes, significantly
more high-quality age and geochemical data are needed for
oceanic hotspot volcanism that gave birth to the mantle plume
hypothesis in the first place. Back
V51B-0571
Upper Mantle
Structure Beneath the Gal\'{a}pagos Hotspot from Surface Wave
Tomography
* Villagomez,
D R (darwin@newberry.uoregon.edu) , Dept. of Geol. Sci., Univ.
of Oregon, Eugene, OR 97403 United States
Toomey, D R (drt@newberry.uoregon.edu) , Dept. of Geol. Sci.,
Univ. of Oregon, Eugene, OR 97403 United States
Hooft, E E (emilie@newberry.uoregon.edu) , Dept. of Geol.
Sci., Univ. of Oregon, Eugene, OR 97403 United States
Solomon, S C (scs@dtm.ciw.edu) , DTM, Carnegie Institution
of Washington, Washington, DC 20015 United States
To understand plume-lithosphere
interaction in a near-ridge setting, we present a surface
wave tomographic study of the upper mantle beneath the Gal\'{a}pagos
Archipelago. We use Rayleigh waves recorded by a network of
10 broadband seismometers deployed from 1999 to 2003 for the
IGUANA experiment and the GSN station PAYG. We analyze waves
in 12 separate frequency bands (8-50 mHz), which are sensitive
to shear wave velocity ({\it Vs}) structure in the upper 150
km. To account for non-great-circle propagation caused by
multipathing we use the two-plane-wave approximation of Forsyth
and others. Two-dimensional models of phase velocity obtained
at each frequency are inverted for three-dimensional variations
in {\it Vs}. Average one-dimensional phase velocities are
1-2% slower than for 0-4 My-old Pacific mantle, and phase
velocities vary laterally by $\pm$3%. Inversions of phase
velocities reveal that {\it Vs} varies regionally from 3.7
to 4.1 km/s, 3-15% slower than predicted along a 1300$\deg$C
adiabat, and that there are two volumes of pronounced low
velocity ($>$10% {\it Vs} reduction). Neither anomaly can
be attributed to temperature alone; instead they require increased
amounts of partial melt. The first anomaly, located beneath
the volcanoes of the southwestern archipelago that erupt large
volumes of enriched magmas, is most pronounced above 40 km
depth and its magnitude increases toward the surface. This
anomaly lies above an area of thinner-than-normal mantle transition
zone and a cylindrical low-velocity body imaged by P and S
wave tomography at depths of 100 to 250 km. This first anomaly
may be the result of melt accumulation above a region of decompression
melting driven by plume upwelling. The second low-velocity
volume underlies the central archipelago, including the islands
of Santiago and Marchena, and appears to be concentrated between
50 and 80 km depth. This anomaly is less pronounced near the
surface, underlies a region that produces MORB, and coincides
with a region of apparent isotropy as reported by Fontaine
and others. This anomaly could indicate decompression melting
of a depleted upper portion of the plume, possibly the result
of modest local upwelling driven by a northward transition
to thinner lithosphere. Our results, together with those from
body wave tomography, suggest that geochemical patterns observed
in the archipelago are in part the result of progressive melting
of material in a plume conduit that rises from southwest to
northeast. We are currently integrating results from surface
and body wave imaging in an effort to constrain interactions
at mantle depths between the hotspot and the Gal\'{a}pagos
Spreading Center. Back
V51B-0572
Young lava
fields on the Cretaceous Pacific Plate in the Japan Trench:
Non-hotspot volcanism?
* Hirano, N (nhirano@geo.titech.ac.jp)
, Department of Earth and Planetary Sciences, Tokyo Institute
of Technology, Ookayama 2-12-1, Meguro, Tokyo, 152-8551 Japan
Haraguchi, S (haraguti@ori.u-tokyo.ac.jp) , Ocean Research
Institute, University of Tokyo, 1-15-1 Minamidai, Nakano,
Tokyo, 164-8639 Japan
Yamamoto, J (jyama@bep.vgs.kyoto-u.ac.jp) , Institute for
Geothermal Sciences, Kyoto University, Noguchibaru, Beppu,
874-0903 Japan
Takahashi, E (etakahas@geo.titech.ac.jp) , Department of Earth
and Planetary Sciences, Tokyo Institute of Technology, Ookayama
2-12-1, Meguro, Tokyo, 152-8551 Japan
Hirata, T (hrt1@geo.titech.ac.jp) , Department of Earth and
Planetary Sciences, Tokyo Institute of Technology, Ookayama
2-12-1, Meguro, Tokyo, 152-8551 Japan
Takahashi, A (ayu@geo.titech.ac.jp) , Department of Earth
and Planetary Sciences, Tokyo Institute of Technology, Ookayama
2-12-1, Meguro, Tokyo, 152-8551 Japan
Ogawa, Y (yogawa@arsia.geo.titech.ac.jp) , Earth and Evolution
Sciences, University of Tsukuba, 1-1-1 Tennodai, Tsukuba,
305-8572 Japan
The northwestern part
of the Pacific Plate is comprised of Early Cretaceous abyssal
oceanic lithosphere and Early to Late Cretaceous seamounts.
Until recently, no present-day volcanic activity had been
definitively documented on the cool, thick, and old Cretaceous
lithosphere; however, Hirano et al. (2001) reported the presence
of anomalously young alkali-basalt lavas (5.95$\pm$0.31 Ma)
on the subducting, $\sim$130 Ma Pacific Plate. The trench-oceanward
slope is characterized by trench-parallel normal faults, resulting
from bending of the subducting Pacific Plate. Some hummock
structures named the Kaiko Knolls can also be observed on
the faulted abyssal plain using seabeam sonar bathymetric
mapping. The Kaiko Knolls hummocks and some of the horst and
graben fault walls are recognized in the seabeam sonar data
by the presence of ocean floor with high acoustic intensity.
The newly discovered lava fields include all hummocks in the
Kaiko Knolls as well as the underlying sheet flow. The distinct
WNW-ESE alignments of knolls are perpendicular to hinge lines
of bending plate of the trench and outer-rise system. Composition
of the dredged lavas shows the garnet presence in the source
because the residual garnet buffered Al$_{2}$O$_{3}$ contents
with degrees of partial melting and lowered HREE contents.
Hirano et al. (2004) demonstrated that the olivine xenocrysts
in this rock were entrained from the uppermost mantle. Volcanic
eruption occurred $\sim$600 km ESE off the northern Japan
Trench based on the radiometric age and the present absolute
motion of the Pacific Plate. Morphological and petrological
evidences show that the magma has been brought to the surface
along some fissures, which can be interpreted along the direction
of the maximum horizontal compression caused by the stress
in the downwarping Pacific Plate at eastern edge of the outer-rise.
Back
V51B-0573
New insights
on the Marquesas volcanic chain emplacement
Adam, C (adam@ipgp.jussieu.fr)
, Institut de Physique du Globe - CNRS, 4,place Jussieu, Paris,
75252 France
* Bonneville, A (bonnevil@ipgp.jussieu.fr) , Institut de Physique
du Globe - CNRS, 4,place Jussieu, Paris, 75252 France
The Marquesas are one
of a number of young hotspot volcanic chains in French Polynesia.
The islands erupted onto 50-65 Ma old seafloor and radiometric
ages of volcanism span from only a few hundred thousand years
for a seamount at the southeast end of the chain to 5.75 Ma
for Eiao atoll at the northwestern end. The Marquesas seem
thus to be a classic hotspot chain however the main orientation
of the chain (N140) does not correspond to the present direction
of the absolute Pacific plate motion (N115). This observation
has intrigued many researchers during the last decades and
we propose here a new explanation based on a precise and complete
mapping of the depth and geoid anomalies associated to the
volcanic alignment. The analysis of the large scale features
of these two datasets allow to characterize the seafloor swell
and the corresponding geoid associated to the volcanic chain
itself but it also evidences to the northeast a N150 elongated
positive geoid anomaly not linked to any seafloor anomaly.
This direction, that corresponds roughly to the direction
of the crustal seafloor magnetic strips, corresponds to the
trend shown by the volcanoes at their creation on the crust.
We propose that the particular orientation of the Marquesas
chain be due to the spreading of a mantle plume head under
the control of the not yet known phenomenum at the origin
of the N150 residual geoid anomaly. Back
V51B-0574
Geochemical
Evolution of the Hikurangi Oceanic Plateau, New Zealand
Hoernle, K (khoernle@ifm-geomar.de)
, Dynamics of the Ocean Floor, IFM-GEOMAR, Wischhofstr. 1-3,
Kiel, 24148 Germany
Hauff, F (fhauff@ifm-geomar.de) , Dynamics of the Ocean Floor,
IFM-GEOMAR, Wischhofstr. 1-3, Kiel, 24148 Germany
* Werner, R (rwerner@ifm-geomar.de) , Tethys Geoconsulting
GmbH, Wischhofstr. 1-3, Kiel, 24148 Germany
Mortimer, N (N.Mortimer@gns.cri.nz) , Insitute of Geological
and Nuclear Sciences, Private Bag 1930, Dunedin, 31-312 New
Zealand
van den Bogaard, P (pbogaard@ifm-geomar.de) , Dynamics of
the Ocean Floor, IFM-GEOMAR, Wischhofstr. 1-3, Kiel, 24148
Germany
Geldmacher, J (jgeldmacher@ifm-geomar.de) , Dynamics of the
Ocean Floor, IFM-GEOMAR, Wischhofstr. 1-3, Kiel, 24148 Germany
Garbe-Schoenberg, D (dgs@gpi.uni-kiel.de) , Institut fuer
Geowissenschaften, Kiel University, Ludewig-Meyn-Str. 10,
Kiel, 24118 Germany
The Hikurangi oceanic
plateau or large igneous province (LIP), located east of the
North Island of New Zealand, covers an area of 350,000 km3
and is located at a depth of 2,500-3,500 b.s.l. The Hikurangi
plateau was possibly connected to the Manihiki LIP (now located
3000 km to the north) but may have been separated by Cretaceous
seafloor spreading at the Osbourn Trough (Billen and Stock,
2000, J. Geophys. Res., 106, 13481-13489). Therefore it may
have formed part of the "greater Ontong Java Plateau event"
(Coffin and Eldholm, Geology, 21, 515-51), the largest magmatic
event preserved on Earth. During the R/V Sonne SO168 ZEALANDIA
cruise, 77 dredge hauls containing igneous samples were recovered
from the Hikurangi Plateau. Volcanic rocks were obtained from
1) the plateau basement along the 1 km high Rapuhia Scarp,
2) large guyot-type seamounts within the plateau, and 3) ridge-type
seamounts associated with rifting of the NE plateau margin
(Hoernle et al., 2004, EOS). The recovered plateau rocks range
from basalts, dolerites and gabbros with tholeiitic and alkali
basaltic to trachybasaltic compositions. The seamount volcanic
rocks have more Si-undersaturated compositions than the plateau
rocks and range from alkali basalts through mugearites to
basanites through tephrites to nephelinites. The plateau basement
rocks have flat rare earth element (REE) patterns similar
to enriched mid-ocean-ridge basalt (MORB) and basement rocks
from other oceanic LIPs, such as Manihiki, Ontong-Java and
the Caribbean. The late-stage seamount lavas show enrichment
in the light REE and all strongly to moderately incompatible
elements, having incompatible element characteristics similar
to the HIMU (high time-integrated U/Pb) component in ocean
island basalts (OIB). Although the Pb isotopic composition
has been extensively effected by seawater alteration, the
freshest samples have enriched (EM-type) Sr-Nd-Pb isotopic
compositions similar to Ontong-Java and Manihiki basement
rocks, suggesting derivation from a common source. In contrast,
the late stage seamount lavas have Sr-Nd-Pb isotopic compositions
similar to HIMU OIB. In conclusion, increasing Si-undersaturation
of the volcanic rocks with decreasing age suggests that the
degree of melting decreased and melting depths increased,
possibly due to increasing lithospheric thickening, during
the waning stages of plateau growth. The change from EM to
HIMU-type trace element and isotopic signatures indicates
that the low-degree, late-stage seamount lavas were derived
from different source material than the large-degree plateau
basement volcanic rocks, either reflecting a heterogeneous
source or distinct sources for the plateau and seamount volcanism.
Ar/Ar age dating is underway. Back
V51B-0575
A South West
Pacific Example of Volcanic Rift Margin Facies in a Backarc
Basin Setting From Norfolk Basin Seismic Reflection Profiles.
Taylor, L (lydia@geosci.usyd.edu.au)
, Sydney University, David Edgeworth Building F05, Eastern
Avenue, University of Sydney, Sydney, NSW 2006 Austria
* Muller, D (dietmarmuller@mac.com) , Sydney University, David
Edgeworth Building F05, Eastern Avenue, University of Sydney,
Sydney, NSW 2006 Austria
Large-volume extrusive
basaltic constructions are ubiquitous along nearly all passive
continental margins formed during the breakup of Pangea, reflecting
rifting above anomalously hot mantle. Here we describe for
the first time a complete passive volcanic conjugate margin
sequence in a back-arc basin, the Norfolk Basin in the Southwest
Pacific, based on high resolution and deep multichannel seismic
reflection profiles from the northern Norfolk Basin FAUST-2
survey and the AGSO law of the sea survey 177. Multichannel
seismic reflection data reveal lava deltas, landward flows,
seaward-dipping reflectors and feeder dikes along the rift
margins and volcanic plateaus within the basin. The timing
of the construction of this volcanic margin is inferred to
be syn-rift due to the the masking of major extensional faults
by volcanic flows. In addition to suggesting high mantle temperatures
beneath the Norfolk Basin, which have been imaged by seismic
tomography (Ritsema et al., 1999), the data provide further
evidence that the Norfolk Basin formed as a backarc basin
in a plume-influenced subduction setting rather than by subduction
driven mantle flow which may explain the apparent landward
jump in back arc spreading. The identification of these volcanic
facies in a backarc setting suggests that volcanic margin
features may be useful indicators to identify plume-arc interactions
in the geological record. Back
V51B-0576
Radial Volcanic
Migrations Above Continental Hotspots: Examples from Arabia
and the Pacific Northwest
* Camp, V E (vcamp@geology.sdsu.edu)
, Dept. of Geological Sciences, San Diego State University,
5500 Campanile Dr., San Diego, CA 92182 United States
Orihashi, Y (oripachi@er.u_tokyo.ac.jp) , Earthquake Research
Inst., University of Tokyo, Yayoi, Bunkyo-ku, Tok;yo, 113-0032
Japan
Ross, M E (m.ross@neu.edu) , Dept. of Geology, Northeastern
University, 14 Homes Hall, Boston, MA 02115 United States
The ongoing debate
on the nature of hotspots has led many to consider alternative,
tectonic models for the origin of hotspot tracks. These linear
volcanic migrations, thought by most to form above supposed
plume tails, have received much attention in the geological
literature. In contrast, radial volcanic migrations forming
above supposed plume heads have gone largely unrecognized.
Two such examples are described here in the harrat volcanic
province of Yemen and Saudi Arabia, and in the Columbia River
Basalt (CRB) Province of the western U.S. Although most models
of plume impingement predict a period of thermal uplift followed
by basalt volcanism, the opposite appears to be true at the
Afar triple junction, where the peak of flood basalt eruption,
from $\sim$31-19 Ma, was followed by uplift and exhumation
which began at $\sim$20 Ma, but accelerated at $\sim$14 Ma.
Uplift here was contemporaneous with the eruption of widely
scattered basaltic lava fields which form the Miocene-to-Holocene
harrat province in Yemen and Saudi Arabia. The Yemeni harrats
become progressively younger and more alkalic away from the
Afar region, prompting Orihashi et al. (1998) to suggest that
they formed by the outward dispersion of a mantle plume, consistent
with high $^{3}$He/$^{4}$He ratios (Ra. 21.6) from the easternmost
of the harrats. Age-equivalent harrats in Saudi Arabia erupted
in a similar fashion, from linear vent systems that become
progressively younger away from the Afar triple junction.
As a group, the Arabian vent systems form a fan-shaped radial
pattern consistent with the outward progression of a hot mantle
source. One striking similarity between the Arabian harrats
and the CRB Province is that both erupted from vents located
on a basement of accreted oceanic terranes, adjacent to an
older cratonic margin. Like the Arabian harrats, the CRB Province
also erupted from a radial system of dikes concentrated along
three, age-progressive trends - the Chief Joseph, Steens-Picture
Gorge, and Northern Nevada Rift trends. These three trends
emanate from a focal point in southeastern Oregon which is
thought to be the Miocene site of plume impingement associated
with the Yellowstone hotspot. Two younger volcanic migrations
emanate from the same region - the eastward-younging Snake
River Plain and the westward-younging Oregon High Lava Plains.
The former is thought to have formed as a hotspot track above
the plume tail, and the latter by asthenospheric drag of the
plume head after it was sheared off against the westward moving
cratonic margin. The recognition of radial, age-progressive
volcanic migrations adds support to the argument that giant
radiating dike systems propagate outward with advancing time.
Such spatial and temporal volcanic and plutonic trends are
consistent with a mantle plume origin, but difficult to reconcile
by nonplume alternatives. Back
V51B-0577
Upper Mantle
Origin of the Newberry Hotspot Track: Evidence From Shear-Wave
Splitting
* Xue, M (meixue@geology.wisc.edu)
, Dept. of Geology and Geophysics, University of Wisconsin,
Madison, 1215 W Dayton St., Madison, WI 53706 United States
Allen, R M (rallen@geology.wisc.edu) , Dept. of Geology and
Geophysics, University of Wisconsin, Madison, 1215 W Dayton
St., Madison, WI 53706 United States
In the northwestern
United States there are two hotspot tracks: the Newberry track
and the Yellowstone track. Both are located on the North American
Plate with the Yellowstone track parallel to plate motion
and the Newberry track oblique to it. While a mantle plume
is probably the most popular cause of the Yellowstone track,
the Newberry track cannot be the product of plate motion over
a stationary mantle source. Instead proposed causal mechanisms
include upper mantle process where melt buoyancy driven convection
is directed west-northwest by subduction-driven corner flow
or alternatively a westward-spreading plume head. In this
SKS splitting study, we collected data from the OATS (Oregon
Array for Teleseismic Study) array, a deployment of the University
of Wisconsin Broadband Network (UWBN) along the Newberry track
from NW to SE Oregon, which was installed in May 2003 and
will operate until September 2005. Measurements were made
for 23 events at 12 OATS stations using Wolfe and Silver's
(1998) multi-event stacking procedure. A gradual rotation
of fast polarization direction is observed from NE-SW at the
northwest end of the array to E-W to the southeast. Most stations
also exhibit null results when the event back azimuth was
parallel or perpendicular to the fast direction determined
from other events, strongly indicating a single layer of anisotropy.
The first order observation is that the SKS splits are not
aligned with the Newberry hotspot track indicating that either
the splits are not sensitive to mantle flow oriented along
the track or the track is not the product of asthenospheric
flow. We prefer the second explanation as our null splitting
observations strongly argue for one layer of anisotropy. If
our continuing analysis confirms this conclusion, then the
alignment of the Yellowstone track with plate motion and anisotropy
may be coincidental rather than representative of the causal
mechanism. Back
V51B-0578
Mantle wedge
perturbation induced by slab detachment and the Mio-Pliocene
bimodal volcanism in the Trans-Mexican Volcanic Belt
* Ferrari, L (luca@geociencias.unam.mx)
, Centro de Geociencias, UNAM, Campus Juriquilla, Qro., Queretaro,
Qro 76230 Mexico
Orozco, M (torozco@geociencias.unam.mx) , Centro de Geociencias,
UNAM, Campus Juriquilla, Qro., Queretaro, Qro 76230 Mexico
Petrone, C M (petrone@geo.unifi.it) , Dipartimento di Scienze
della Terra, Universit di Firenze, Via La Pira 4, Firenze,
50121 Italy
The trench-oblique
orientation, the coexistence of geochemical diverse lavas
(OIB, CAB, Adakites etc.), and the absence of seismicity beneath
the Trans-Mexican Volcanic Belt (TMVB) prompted several workers
to formulate genetic models at variance with a classic subduction
scenario, including the presence of a mantle plume beneath
central Mexico. Based on a careful analysis of its geologic
and geochemical evolution we consider, in turn, that this
volcanic chain is a continental arc whose complexity is due
to the thermal and mechanical perturbation of the mantle wedge
imposed by plate history and modulated by crustal thickness,
composition and structures. The TMVB began in Middle Miocene
as a WNW trending arc of andesitic-dacitic polygenetic volcanoes.
This relatively normal situation changed in Late Miocene,
when mafic plateaus, cinder cones and fissural lava flows
were emplaced to the north of the present TMVB with a clear
eastward migrating pattern from ~11.5 and 6.5 Ma. This mafic
pulse has been related to the eastward propagation of a slab
detachment episode, in the southern Gulf of California, which
produced a transient thermal anomaly in the mantle (Ferrari,
2004, Geology). Following this episode, volcanism strongly
decreased and becomes more evolved. Dacitic to rhyolitic domes
and ignimbrites were emplaced in a belt located just to the
south of the previous episode between 7.5 and ~3.0 Ma. Dome
complexes dominate the western half of the TMVB, whereas caldera-forming
ignimbrites are common to the east. The geochemical character
of this bimodal volcanism was analyzed using new Sr and Nd
isotope and our database of chemical data (~3,000 samples).
The mafic pulse has a basaltic composition [average SiO2 =
50.7±4.0(1s)] readily distinguishable from the following
volcanism. Most basalts have a subduction signature, although
they show no systematic correlation with distance from the
trench. East of Long. 99° W, however, they are Ne-normative
and display much lower to none influence of subducted sediments
and fluids (lower Ba/Nb, La/Nb and Th/Nb). This geochemical
boundary separates the region of Oligo-Miocene subduction
metasomatism related to the Sierra Madre Occidental (to the
west) from the region where the mantle was unaffected by subduction
since the Permian. For the western TMVB rhyolites, the available
isotope data (87Sr/86Sr = 0.70396-0.70597; εNd = 4.07-5.01)
point to a mantle origin with variable crust assimilation.
This suggests that the latest Miocene switch of volcanism
toward more silicic composition was the effect of the decrease
in subduction rate of the Rivera plate (DeMets & Traylen
2000), an expected consequence of the loss of slab pull after
slab detachment. Decrease in convergence reduced flux of the
mantle and amount of melting, so the magma started to pond
in the crust and underwent fractional crystallization and
variable assimilation. In the eastern half of the TMVB both
basalts and rhyolites show the highest signature of crustal
contamination in the 87Sr/86Sr vs. 143Nd/144Nd plane. This
region corresponds to the area where crust is thicker and
extension was much less intense than in the west or absent.
Here partial melting of the crust may play an important role
in generating the dacitic to rhyolitic magmas, likely as a
consequence of the rollback of the slab that exposed the base
of the upper plate to hotter asthenosphere. Back
V51B-0579
Why are Low-Ti
Basalts of the Siberian Traps Large Igneous Province Similar
to Island Arc Basalts?
* Ivanov, A V
(aivanov@crust.irk.ru) , Institute of the Earth's Crust SB
RAS, Lermontov st. 128, Irkutsk, 664033 Russian Federation
Rasskazov, S V (rassk@crust.irk.ru) , Institute of the Earth's
Crust SB RAS, Lermontov st. 128, Irkutsk, 664033 Russian Federation
Demonterova, E I (dem@crust.irk.ru) , Institute of the Earth's
Crust SB RAS, Lermontov st. 128, Irkutsk, 664033 Russian Federation
Yasnygina, T A (rassk@crust.irk.ru) , Institute of the Earth's
Crust SB RAS, Lermontov st. 128, Irkutsk, 664033 Russian Federation
Maslovskay, M N (rassk@crust.irk.ru) , Institute of the Earth's
Crust SB RAS, Lermontov st. 128, Irkutsk, 664033 Russian Federation
Feoktistov, G D (rassk@crust.irk.ru) , Institute of the Earth's
Crust SB RAS, Lermontov st. 128, Irkutsk, 664033 Russian Federation
Tholeiitic and alkaline
basalts are predominant rock types in the Late Permian - Early
Triassic Siberian Traps Large Igneous Province (STLIP). These
basalts belong to high-Ti and low-Ti series of rocks. A peculiarity
of the low-Ti basalts is the virtual similarity with typical
island arc basalts. On a primitive mantle normalized diagram,
both the low-Ti basalts of the STLIP and island arc basalts
exhibit prominent depletion of Th, Ta-Nb, Pr and Sr relative
neighboring elements. As an example, the low-Ti basalts are
characterized by almost the same trace element abundances
and compositional trends as basalts from Klyuchevskoi volcano,
which belongs to the modern volcanic arc of Kamchatka. Using
ratios of element pairs with similar rock-melt distribution
coefficients such as Sr-Pr, Nb-U, K-Nb and Ce-Pb we infer
three principal components: 1) oceanic sediments (or upper
crust), 2) melts of oceanic basalt type (either middle oceanic
ridge or oceanic island basalt types), and 3) melts of island
arc basalt type (or lower crust). Basalts of the high-Ti and
low-Ti series of the STLIP form two nearly perpendicular trends
between the first and second and between the first and third
components. Such trends, and especially the approaching of
the trends towards the same component, which is similar to
oceanic sediments cannot be explained by a model of plume-lithosphere
interaction. An impact-induced melting model could explain
the trace-element data satisfactorily, but it can also be
rejected because volcanism of the STLIP initiated in the Late
Permian, a few Ma before the Permo-Triassic stratigraphic
boundary. Prolonged subduction beneath the Siberian part of
Pangea occurred in the Permian and Triassic. Remnants of the
Mongolo-Okhotsk slab beneath Siberia are still visible in
seismic tomography images. Therefore, we suggest that the
origin of the STLIP is related to subduction processes. Melts
from subducted sediment-bearing oceanic crust played a major
role in triggering magmatic processes in the sublithospheric
and lithospheric upper mantle. The low-Ti basalts were produced
within the sublithospheric upper mantle, which was highly
metasomatized by subduction-derived fluids. This explains
both the extremely large volume of melts and trace element
similarity with island arc basalts. Back
http://www.mantleplumes.org
V51B-0580
Silicate Veining
Above an Ascending Mantle Plume - Evidence from New Ethiopian
Xenolith Localities
* Rooney, T O
(tor102@psu.edu) , The Pennsylvania State University, Dept.
of Geosciences, The Pennsylvania State University, University
Park, PA 16802 United States
Furman, T (furman@geosc.psu.edu) , The Pennsylvania State
University, Dept. of Geosciences, The Pennsylvania State University,
University Park, PA 16802 United States
Ayalew, D (dereayal@geol.aau.edu.et) , Addis Ababa University,
Dept. of Geology and Geophysics, Addis Ababa University, P.O.
Box 1176, Addis Ababa, 0000 Ethiopia
Yirgu, G (yirgu.g@geol.aau.edu.et) , Addis Ababa University,
Dept. of Geology and Geophysics, Addis Ababa University, P.O.
Box 1176, Addis Ababa, 0000 Ethiopia
Quaternary basaltic
eruptions in the Debre Zeyit (Bishoftu) and Butajira regions
of the Main Ethiopian Rift host Al-augite, norite and rare
lherzolite xenoliths, xenocrysts and megacrysts. These explosive
basaltic eruptions are located 20 km to the west of the main
rift axis and are characterized by cinder cones and maars.
The host basalt was generated as a small degree partial melt
of fertile peridotite between 15 and 25 kb and host abundant
Al-augite (Type II) xenoliths derived from pressures up to
10 kb. The central Main Ethiopian Rift lies in a transitional
zone between the continental rifting of East Africa and the
sea floor spreading associated with the Red Sea. Lithospheric
and sub-lithospheric processes that occur during the transition
from continental to oceanic magmatism may be investigated
using these xenolith-bearing basalts. Neither carbonatitic
nor hydrous (amphibole + phlogopite) metasomatism is evident
in either the xenoliths or host basalts, suggesting that infiltration
of silicate melts that produced Al-augite veining dominates
the regional lower crust and lithospheric mantle. These veins
are significantly hotter (200 - 300 $\deg$C) than the lherzolite
wall rock they intrude suggesting the thermal influence of
the Afar plume. Recent geophysical tomography indicates that
this veining is pervasive and segmented, supporting the association
of these Al-augite veins with the formation of a proto-ridge
axis. Al-augite xenoliths and megacrysts have been observed
in other continental rift settings such as Durango (Luhr,
2001) and Lake Baikal (Litasov, 2000), indicating Al-augite
silicate melt metasomatism is a fundamental process associated
with continental rift development. Back
V51B-0581
Antipodal Hotspots
and Bipolar Catastrophes: Were Oceanic Large-Body Impacts
to Blame?
* Hagstrum, J
T (jhag@usgs.gov) , U.S. Geological Survey, 345 Middlefield
Road, MS 937, Menlo Park, CA 94025 United States
One aspect of the hotspot
distribution that has received little attention is its antipodal
character. Of 45 "primary" hotspots, found in most hotspot
compilations, 22 (49%) form antipodal pairs within conservative
drift limits ($\leq$20 mm/yr). All but 4 of the remaining
primary hotspots have volcanic centers near their antipodes.
In addition, the available ages, or estimated minimum age
ranges, for both hotspots of an antipodal pair tend to be
similar ($\leq$10 Myr difference) or overlap. Monte Carlo
analyses indicate that the primary antipodal hotspot pairs
and their ages are not due to chance at the $>$99.9% confidence
level ({\it p}$<$0.001). All hotspot pairs include at least
one oceanic hotspot, and these are consistently opposite those
hotspots related to large igneous provinces and continental
volcanism. A model of hotspot formation is proposed in which
minor volcanism is induced at, and lithospheric fracturing
and flood-basalt volcanism is caused by focused seismic energy
antipodal to, oceanic large-body impacts. Because continental
impacts have low seismic efficiencies ($\sim$10$^{-4}$), continents
possibly acted as shields to the formation of antipodal hotspot
pairs. Published numerical models indicate that large oceanic
impacts ($\sim$10-km-diameter bolide) penetrate well into
the upper mantle ($\sim$40-km depth), eject mostly water or
water vapor from the transient crater, and generate megatsunami
($\sim$4 km initial height) capable of coastal stratigraphic
effects on a global scale. Impact-generated megatsunami, consequently,
are expected to leave the most prominent and widespread record
of large oceanic impacts, and might have been responsible
for apparent rapid eustatic changes in sea level and abrupt
changes in the isotopic composition of seawater in the geologic
past. Moreover, large oceanic impacts during the Late Permian
were perhaps the principal cause of end-Kazanian and end-Tatarian
flood basalt eruptions, apparent regressive-transgressive
shifts in sea level, and pulses in extinction rates making
up the Permian/Triassic transition, and might have initiated
the Cretaceous/Tertiary transition at $\sim$68-67 Ma. Phanerozoic
mass extinction events, therefore, might have been the result
of catastrophic megatsunami in a dominantly oceanic hemisphere
and vast quantities of noxious volcanic gases in a dominantly
continental one. Back
V51B-0582
Mantle Plume
Magmatism on Present-day Mars
* Kiefer, W S
(kiefer@lpi.usra.edu) , Lunar and Planetary Institute, 3600
Bay Area Blvd., Houston, TX 77058 United States
Two independent types
of evidence demonstrate the existence of very young volcanism
on Mars. The shergottites are a type of igneous meteorite
from Mars, many of which have radiometric ages of just 180
million years. High resolution images of some lava flows have
such a paucity of small impact craters that the flows must
be quite young, perhaps just 10-30 million years. The concentrated
nature of young volcanic activity in just two provinces of
Mars, Tharsis and Elysium, is best understood as a result
of upwelling mantle plumes which originate deep in the martian
mantle. Each plume feeds a single large volcano, such as Olympus
Mons. Thus, Tharsis consists of several distinct plumes, set
within a broader zone of internally heated upwelling. Numerical
models of plume magmatism have been developed which use melting
relationships appropriate for martian mantle compositions,
as inferred from the shergottite meteorites. These models
can explain the geologically inferred magma production rate
and the geochemically inferred mean melt fraction, provided
that the martian mantle has retained about half of its original
content of radioactive elements, with the remainder of the
heat production now in the crust. The recently recognized
shergottite Yamato 980459 is more magnesian than previously
known martian meteorites and has a significantly higher melting
temperature. The required high temperature further enhances
the requirement for hot mantle plumes on Mars. Previous models
of martian plume volcanism assumed a depth-dependent rheology.
In these models, the thickness of the upper, high viscosity
layer was adjusted to produce a heat flux that is consistent
with the elastic lithosphere thickness inferred from gravity
modeling. New models are now in development using a more realistic,
temperature-dependent olivine rheology. The improved rheology
model may modify previous results in several ways. The low
viscosity in the plume conduit will permit faster ascent of
material through the mantle and may reduce the amount of cooling
of the plume by the surrounding mantle. Also, the new models
will permit local thinning of the lithosphere in the center
of the plume. These effects may be crucial in explaining the
high melting temperature of Yamato 980459. Back
http://www.lpi.usra.edu/science/kiefer/home.html
|